xgettext: Python: Add test for warning that was broken in 0.21.x and 0.22.x.
Reported by Max Smolens <msmolens@gmail.com> in
<https://lists.gnu.org/archive/html/bug-gettext/2020-10/msg00001.html>.
* gettext-tools/tests/xgettext-python-9: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add it.