* test_improvements - Add __init__.py files to tests dirs so you can run all tests
* test_improvements - added helpers file, restructured tests to work without previous data and to be grouped with api type
* http_retries - Implement Retry for all requests
* Readme - Update README with contributing info
* PR Feedback - Added CONTRIBUTING.md, fixed README, fixed test
* 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.
