Logging test fail on python 3.8+

[kudlatyamroth]

Since python 3.8 logger format validates format by default: https://docs.python.org/3/library/logging.html#logging.Formatter

In tests there is config with failing format:
https://github.com/powerline/powerline/blob/develop/tests/test_python/test_logging.py#L323
https://github.com/powerline/powerline/blob/develop/tests/test_python/test_logging.py#L343
https://github.com/powerline/powerline/blob/develop/tests/test_python/test_logging.py#L344

these format not include any %(message)s or any other placeholder.

Correct way would be to add placesholder in tests and fix checks, but if someone is using powerline on python older then 3.8 and have setting with incorect format, then this is a breaking change for them

the other way that will not introduce any breaking changes will be to add validate=False in:
https://github.com/powerline/powerline/blob/develop/powerline/__init__.py#L255

Maybe there should be github workflow for running these tests on every PR, and tests should include newest python always :)