TST: add github test action

[hagenw]

This adds a definition file for executing tests on github (so called github Actions).
It updates the pytest settings in setup.cfg to store the coverage report as a XML file, that is then uploaded in the Action to codecov.io which we can then use in other pull request to report if the code coverage has changed.
It also limits the installed flake8 version as the newly released 5.0.0 version does no longer work with the current latest pytest version.

Tests are added for:

• Ubuntu, Python 3.7, 3.8, 3.9
• Windows, Python 3.8
• MacOS, Python 3.8
• Ubuntu and building docs, Python 3.8

[sandneurodata]

all good to me . Just a quick question regarding audb : why did you change it from a global import to an import within a function in test_audeering_internal?

[hagenw]

For whatever reason pytest executes all imports in the files starting with test_ inside the tests/ folder, including test_audeering_internal.py and ignore that this file is not marked to be executed.
In order to not have to install audb for the tests, I solved this problem by moving the import inside the function.
Maybe we can get rid of it altogether by removing test_audeering_internal.py altogether, see #2.