[3.12] gh-110519: Improve deprecation warning in the gettext module (GH-110520) (GH-110563)

Deprecation warning about non-integer numbers in gettext now always refers
to the line in the user code where gettext function or method is used.
Previously, it could refer to a line in gettext code.

Also, increase test coverage for NullTranslations and domain-aware functions
like dngettext().
(cherry picked from commit 326c6c4e07137b43c49b74bd5528619360080469)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>