Other answers here seem to indicate that constructing this list programmatically is difficult and fraught with traps. However, doing so is probably unnecessary since the documentation contains a complete list of the standard encodings Python supports, and has done since Python 2.3.
You can find these lists (for each stable version of the language so far released) at:
Below are the lists for each documented version of Python. Note that if you want backwards-compatibility rather than just supporting a particular version of Python, you can just copy the list from the latest Python version and check whether each encoding exists in the Python running your program before trying to use it.
Python 2.3 (59 encodings)
['ascii',
'cp037',
'cp424',
'cp437',
'cp500',
'cp737',
'cp775',
'cp850',
'cp852',
'cp855',
'cp856',
'cp857',
'cp860',
'cp861',
'cp862',
'cp863',
'cp864',
'cp865',
'cp869',
'cp874',
'cp875',
'cp1006',
'cp1026',
'cp1140',
'cp1250',
'cp1251',
'cp1252',
'cp1253',
'cp1254',
'cp1255',
'cp1256',
'cp1257',
'cp1258',
'latin_1',
'iso8859_2',
'iso8859_3',
'iso8859_4',
'iso8859_5',
'iso8859_6',
'iso8859_7',
'iso8859_8',
'iso8859_9',
'iso8859_10',
'iso8859_13',
'iso8859_14',
'iso8859_15',
'koi8_r',
'koi8_u',
'mac_cyrillic',
'mac_greek',
'mac_iceland',
'mac_latin2',
'mac_roman',
'mac_turkish',
'utf_16',
'utf_16_be',
'utf_16_le',
'utf_7',
'utf_8']
Python 2.4 (85 encodings)
['ascii',
'big5',
'big5hkscs',
'cp037',
'cp424',
'cp437',
'cp500',
'cp737',
'cp775',
'cp850',
'cp852',
'cp855',
'cp856',
'cp857',
'cp860',
'cp861',
'cp862',
'cp863',
'cp864',
'cp865',
'cp866',
'cp869',
'cp874',
'cp875',
'cp932',
'cp949',
'cp950',
'cp1006',
'cp1026',
'cp1140',
'cp1250',
'cp1251',
'cp1252',
'cp1253',
'cp1254',
'cp1255',
'cp1256',
'cp1257',
'cp1258',
'euc_jp',
'euc_jis_2004',
'euc_jisx0213',
'euc_kr',
'gb2312',
'gbk',
'gb18030',
'hz',
'iso2022_jp',
'iso2022_jp_1',
'iso2022_jp_2',
'iso2022_jp_2004',
'iso2022_jp_3',
'iso2022_jp_ext',
'iso2022_kr',
'latin_1',
'iso8859_2',
'iso8859_3',
'iso8859_4',
'iso8859_5',
'iso8859_6',
'iso8859_7',
'iso8859_8',
'iso8859_9',
'iso8859_10',
'iso8859_13',
'iso8859_14',
'iso8859_15',
'johab',
'koi8_r',
'koi8_u',
'mac_cyrillic',
'mac_greek',
'mac_iceland',
'mac_latin2',
'mac_roman',
'mac_turkish',
'ptcp154',
'shift_jis',
'shift_jis_2004',
'shift_jisx0213',
'utf_16',
'utf_16_be',
'utf_16_le',
'utf_7',
'utf_8']
Python 2.5 (86 encodings)
['ascii',
'big5',
'big5hkscs',
'cp037',
'cp424',
'cp437',
'cp500',
'cp737',
'cp775',
'cp850',
'cp852',
'cp855',
'cp856',
'cp857',
'cp860',
'cp861',
'cp862',
'cp863',
'cp864',
'cp865',
'cp866',
'cp869',
'cp874',
'cp875',
'cp932',
'cp949',
'cp950',
'cp1006',
'cp1026',
'cp1140',
'cp1250',
'cp1251',
'cp1252',
'cp1253',
'cp1254',
'cp1255',
'cp1256',
'cp1257',
'cp1258',
'euc_jp',
'euc_jis_2004',
'euc_jisx0213',
'euc_kr',
'gb2312',
'gbk',
'gb18030',
'hz',
'iso2022_jp',
'iso2022_jp_1',
'iso2022_jp_2',
'iso2022_jp_2004',
'iso2022_jp_3',
'iso2022_jp_ext',
'iso2022_kr',
'latin_1',
'iso8859_2',
'iso8859_3',
'iso8859_4',
'iso8859_5',
'iso8859_6',
'iso8859_7',
'iso8859_8',
'iso8859_9',
'iso8859_10',
'iso8859_13',
'iso8859_14',
'iso8859_15',
'johab',
'koi8_r',
'koi8_u',
'mac_cyrillic',
'mac_greek',
'mac_iceland',
'mac_latin2',
'mac_roman',
'mac_turkish',
'ptcp154',
'shift_jis',
'shift_jis_2004',
'shift_jisx0213',
'utf_16',
'utf_16_be',
'utf_16_le',
'utf_7',
'utf_8',
'utf_8_sig']
Python 2.6 (90 encodings)
['ascii',
'big5',
'big5hkscs',
'cp037',
'cp424',
'cp437',
'cp500',
'cp737',
'cp775',
'cp850',
'cp852',
'cp855',
'cp856',
'cp857',
'cp860',
'cp861',
'cp862',
'cp863',
'cp864',
'cp865',
'cp866',
'cp869',
'cp874',
'cp875',
'cp932',
'cp949',
'cp950',
'cp1006',
'cp1026',
'cp1140',
'cp1250',
'cp1251',
'cp1252',
'cp1253',
'cp1254',
'cp1255',
'cp1256',
'cp1257',
'cp1258',
'euc_jp',
'euc_jis_2004',
'euc_jisx0213',
'euc_kr',
'gb2312',
'gbk',
'gb18030',
'hz',
'iso2022_jp',
'iso2022_jp_1',
'iso2022_jp_2',
'iso2022_jp_2004',
'iso2022_jp_3',
'iso2022_jp_ext',
'iso2022_kr',
'latin_1',
'iso8859_2',
'iso8859_3',
'iso8859_4',
'iso8859_5',
'iso8859_6',
'iso8859_7',
'iso8859_8',
'iso8859_9',
'iso8859_10',
'iso8859_13',
'iso8859_14',
'iso8859_15',
'iso8859_16',
'johab',
'koi8_r',
'koi8_u',
'mac_cyrillic',
'mac_greek',
'mac_iceland',
'mac_latin2',
'mac_roman',
'mac_turkish',
'ptcp154',
'shift_jis',
'shift_jis_2004',
'shift_jisx0213',
'utf_32',
'utf_32_be',
'utf_32_le',
'utf_16',
'utf_16_be',
'utf_16_le',
'utf_7',
'utf_8',
'utf_8_sig']
Python 2.7 (93 encodings)
['ascii',
'big5',
'big5hkscs',
'cp037',
'cp424',
'cp437',
'cp500',
'cp720',
'cp737',
'cp775',
'cp850',
'cp852',
'cp855',
'cp856',
'cp857',
'cp858',
'cp860',
'cp861',
'cp862',
'cp863',
'cp864',
'cp865',
'cp866',
'cp869',
'cp874',
'cp875',
'cp932',
'cp949',
'cp950',
'cp1006',
'cp1026',
'cp1140',
'cp1250',
'cp1251',
'cp1252',
'cp1253',
'cp1254',
'cp1255',
'cp1256',
'cp1257',
'cp1258',
'euc_jp',
'euc_jis_2004',
'euc_jisx0213',
'euc_kr',
'gb2312',
'gbk',
'gb18030',
'hz',
'iso2022_jp',
'iso2022_jp_1',
'iso2022_jp_2',
'iso2022_jp_2004',
'iso2022_jp_3',
'iso2022_jp_ext',
'iso2022_kr',
'latin_1',
'iso8859_2',
'iso8859_3',
'iso8859_4',
'iso8859_5',
'iso8859_6',
'iso8859_7',
'iso8859_8',
'iso8859_9',
'iso8859_10',
'iso8859_11',
'iso8859_13',
'iso8859_14',
'iso8859_15',
'iso8859_16',
'johab',
'koi8_r',
'koi8_u',
'mac_cyrillic',
'mac_greek',
'mac_iceland',
'mac_latin2',
'mac_roman',
'mac_turkish',
'ptcp154',
'shift_jis',
'shift_jis_2004',
'shift_jisx0213',
'utf_32',
'utf_32_be',
'utf_32_le',
'utf_16',
'utf_16_be',
'utf_16_le',
'utf_7',
'utf_8',
'utf_8_sig']
Python 3.0 (89 encodings)
['ascii',
'big5',
'big5hkscs',
'cp037',
'cp424',
'cp437',
'cp500',
'cp737',
'cp775',
'cp850',
'cp852',
'cp855',
'cp856',
'cp857',
'cp860',
'cp861',
'cp862',
'cp863',
'cp864',
'cp865',
'cp866',
'cp869',
'cp874',
'cp875',
'cp932',
'cp949',
'cp950',
'cp1006',
'cp1026',
'cp1140',
'cp1250',
'cp1251',
'cp1252',
'cp1253',
'cp1254',
'cp1255',
'cp1256',
'cp1257',
'cp1258',
'euc_jp',
'euc_jis_2004',
'euc_jisx0213',
'euc_kr',
'gb2312',
'gbk',
'gb18030',
'hz',
'iso2022_jp',
'iso2022_jp_1',
'iso2022_jp_2',
'iso2022_jp_2004',
'iso2022_jp_3',
'iso2022_jp_ext',
'iso2022_kr',
'latin_1',
'iso8859_2',
'iso8859_3',
'iso8859_4',
'iso8859_5',
'iso8859_6',
'iso8859_7',
'iso8859_8',
'iso8859_9',
'iso8859_10',
'iso8859_13',
'iso8859_14',
'iso8859_15',
'johab',
'koi8_r',
'koi8_u',
'mac_cyrillic',
'mac_greek',
'mac_iceland',
'mac_latin2',
'mac_roman',
'mac_turkish',
'ptcp154',
'shift_jis',
'shift_jis_2004',
'shift_jisx0213',
'utf_32',
'utf_32_be',
'utf_32_le',
'utf_16',
'utf_16_be',
'utf_16_le',
'utf_7',
'utf_8',
'utf_8_sig']
Python 3.1 (90 encodings)
['ascii',
'big5',
'big5hkscs',
'cp037',
'cp424',
'cp437',
'cp500',
'cp737',
'cp775',
'cp850',
'cp852',
'cp855',
'cp856',
'cp857',
'cp860',
'cp861',
'cp862',
'cp863',
'cp864',
'cp865',
'cp866',
'cp869',
'cp874',
'cp875',
'cp932',
'cp949',
'cp950',
'cp1006',
'cp1026',
'cp1140',
'cp1250',
'cp1251',
'cp1252',
'cp1253',
'cp1254',
'cp1255',
'cp1256',
'cp1257',
'cp1258',
'euc_jp',
'euc_jis_2004',
'euc_jisx0213',
'euc_kr',
'gb2312',
'gbk',
'gb18030',
'hz',
'iso2022_jp',
'iso2022_jp_1',
'iso2022_jp_2',
'iso2022_jp_2004',
'iso2022_jp_3',
'iso2022_jp_ext',
'iso2022_kr',
'latin_1',
'iso8859_2',
'iso8859_3',
'iso8859_4',
'iso8859_5',
'iso8859_6',
'iso8859_7',
'iso8859_8',
'iso8859_9',
'iso8859_10',
'iso8859_13',
'iso8859_14',
'iso8859_15',
'iso8859_16',
'johab',
'koi8_r',
'koi8_u',
'mac_cyrillic',
'mac_greek',
'mac_iceland',
'mac_latin2',
'mac_roman',
'mac_turkish',
'ptcp154',
'shift_jis',
'shift_jis_2004',
'shift_jisx0213',
'utf_32',
'utf_32_be',
'utf_32_le',
'utf_16',
'utf_16_be',
'utf_16_le',
'utf_7',
'utf_8',
'utf_8_sig']
Python 3.2 (92 encodings)
['ascii',
'big5',
'big5hkscs',
'cp037',
'cp424',
'cp437',
'cp500',
'cp720',
'cp737',
'cp775',
'cp850',
'cp852',
'cp855',
'cp856',
'cp857',
'cp858',
'cp860',
'cp861',
'cp862',
'cp863',
'cp864',
'cp865',
'cp866',
'cp869',
'cp874',
'cp875',
'cp932',
'cp949',
'cp950',
'cp1006',
'cp1026',
'cp1140',
'cp1250',
'cp1251',
'cp1252',
'cp1253',
'cp1254',
'cp1255',
'cp1256',
'cp1257',
'cp1258',
'euc_jp',
'euc_jis_2004',
'euc_jisx0213',
'euc_kr',
'gb2312',
'gbk',
'gb18030',
'hz',
'iso2022_jp',
'iso2022_jp_1',
'i