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

[Martin v. Löwis]

Martin v. Löwis <martin at v.loewis.de> added the comment:

The gettext module is intentionally written in pure Python; it should
stay that way. Whether or not the _locale module should also grow
support for pgettext is a different issue.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue2504>
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