gh-104400: Remove ``fintl.gettext`` from pygettext (#129580)

The ``fintl`` module is never installed or tested, meaning that the
fallback identity function is unconditionally used for ``_()``.
This means we can simplify, converting the docstring to a real
docstring, and converting some other strings to f-strings.

We also convert the module to UTF-8, sort imports,
and remove the history comment, which was last updated in 2002.
Consult the git history for a more accurate summary of changes.