* Linting of OAuth2 mod. + update tests to detect core package path
* Tweak imports in tests + update tox ini to ignore Flake8 E501 error
* Tweak tox ini - ignore examples
* Remove `f`-string from OAuth2 mod.
* More import tweaks in core package + tests
* Update flake8 config. in tox ini - set line length limit to 99 chars.