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

[Cheryl Sabella]

Cheryl Sabella <cheryl.sabella at gmail.com> added the comment:

I've created a pull request for this based on Éric's patch in 2010.  I attempted to incorporate the changes from #29755 into the new functions and tests.  I believe these changes take care of items 1 and 2 from msg122439.

I haven't tried to address item 3 in msg122439 yet.

Since this is already a large change, item 4 in msg122439 might be better as its own issue.

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stage: patch review -> needs patch

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