Update test dependencies: aioxmpp, pytest-asyncio + add pytz to requirements.txt

This fixes the following error when running the tests:
E   ModuleNotFoundError: No module named 'pytz'

Instead of pytz only pytz-deprecation-shim was installed.

TODO Check if "pytz" can be removed from requirements.txt later on.

tests/requirements.txt

@@ -1,3 +1,4 @@
-aioxmpp==0.13.2
+aioxmpp==0.13.3
 pip-chill==1.0.1
-pytest-asyncio==0.18.3
+pytest-asyncio==0.21.0
+pytz==2022.7.1