gh-118761: Improve the import time of ``gettext`` (#128898)

``gettext`` is often imported in programs that may not end up translating
anything. In fact, the ``struct`` module already has a delayed import when
parsing ``GNUTranslations`` to speed up the no ``.mo`` files case. The re module
is also used in the same situation, but behind a function chain only
called by ``GNUTranslations``.

Cache the compiled regex globally the first time it is used. The
finditer function is converted to a method call on the compiled
object which is slightly more efficient, and necessary for the
delayed re import.