plural.py in Pywikibot-compat was derived by @Xqt from plural-gettext.txt in commit a3078ff34a ; and was copied to Pywikibot in bfb857199 (minor fix minutes later 5664c76) , with no substantive changes since August 2011.
That was last updated in August 2011 from http://translate.sourceforge.net/wiki/l10n/pluralforms (rETRA56bf77b78d)
That project is now at http://localization-guide.readthedocs.org/en/latest/l10n/pluralforms.html and is now hosted at https://github.com/translate/l10n-guide/blob/master/docs/l10n/pluralforms.rst and the raw data is in https://github.com/translate/translate/blob/master/translate/lang/data.py#L25 . The history shows many plural fixes since 2011.
The 'proper' source now is CLDR
With regards to MediaWiki-extensions-Translate , it is only GettextFFS which uses the plural-gettext.txt data file - MediaWiki itself now uses languages/data/plurals.xml and languages/data/plurals-mediawiki.xml which are CLDR based, and I guess the majority of the Translate extension uses this also. (scripts/plural-comparison.php also uses the plural-gettext.txt data file, but only to compare the various plural data )
However I expect that plural-gettext.txt distributed by MediaWiki-extensions-Translate should be updated.
>>> from translate.lang import data >>> data.languages['gu'] ('Gujarati', 2, '(n != 1)')