bpo-2504: Add pgettext() and variants to gettext

[csabella]

Original patch by Franz Glasner.

https://bugs.python.org/issue2504

[damufo]

Hello:
How is this?
I think it would be interesting to complete it.
Being able to use context in gettext would allow for example to specify the gender.

[serhiy-storchaka]

Remove l*gettext() functions and fix the documentation.

[bedevere-bot]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[csabella]

I have made the requested changes; please review again.

[bedevere-bot]

Thanks for making the requested changes!

@serhiy-storchaka: please review the changes made to this pull request.

[serhiy-storchaka]

Please add also a What's New entry.