[issue2504] Add gettext.pgettext() and variants support

[Serhiy Storchaka]

Serhiy Storchaka <storchaka+cpython at gmail.com> added the comment:

l*gettext() functions have been just deprecated in issue33710. Please update the PR and remove new l*gettext() functions.

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versions: +Python 3.8 -Python 3.7

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