Drop Python 2.7 support

[orsinium]

1. Drop support for all outdated Python versions. The oldest officially supported Python version is 3.6 which reaches EoL on 26 December 2021 (which is quite soon).
2. Add in tox and CI checks Python versions 3.6 - 3.9. I've checked it with 3.9 locally, it works.

No changes in the code have been made. Still, this is an important step that will simplify #146 and #149. Python 2.7 is out of support for almost a year, It's time to make this step.

[pgajdos]

Eventually please consider to drop mock dependency, or make it optional at least.

[jsocol]

It's dropped from the new CI config! Thanks for opening and raising this.