pytest 7.1.0 does not discover conftest.py

[david-zwicker]

The latest pytest version does have a bug where conftest.py is not discovered properly: pytest-dev/pytest#9767
This breaks our tests, so we temporarily fixed the pytest version to 7.0.1 in tests/requirements.txt and .github/workflows/tests_minversion.yml.
This change should be reverted once pytest is fixed.