python: Workaround for a regression in GnuPG 2.2.21

* lang/python/tests/support.py (is_gpg_version): New.
* lang/python/tests/t-encrypt-sym.py: Add workaround.

• GnuPG-bug-id: T4991
• Signed-off-by: Werner Koch <wk@gnupg.org>