Deprecation warnings from CI

[stefan6419846]

The latest CI run indicates some deprecations (see https://app.travis-ci.com/github/Afonasev/flake8-plugin-utils):

$ make lint test BIN=""
flake8 --jobs 4 --statistics --show-source flake8_plugin_utils tests
W0503: Deprecated config options were used: not_skip.Please see the 5.0.0 upgrade guide: bit.ly/isortv5.
W0503: Deprecated config options were used: not_skip.Please see the 5.0.0 upgrade guide: bit.ly/isortv5.
W0503: Deprecated config options were used: not_skip.Please see the 5.0.0 upgrade guide: bit.ly/isortv5.
W0503: Deprecated config options were used: not_skip.Please see the 5.0.0 upgrade guide: bit.ly/isortv5.

[...]

../../../virtualenv/python3.6.7/lib/python3.6/site-packages/_pytest/config/__init__.py:1184

  /home/travis/virtualenv/python3.6.7/lib/python3.6/site-packages/_pytest/config/__init__.py:1184: PytestDeprecationWarning: The --strict option is deprecated, use --strict-markers instead.

    _pytest.deprecated.STRICT_OPTION, stacklevel=2

-- Docs: https://docs.pytest.org/en/stable/warnings.html

The isort issues probably can be fixed by just removing the corresponding option, see https://pycqa.github.io/isort/docs/upgrade_guides/5.0.0.html#not_skip. The pytest issue is documented at https://docs.pytest.org/en/7.0.x/deprecations.html#the-strict-command-line-option.