Drop python 2.7
This should speed up CI and make code reviewing simpler (hopefully!)
#662 CI: Only update docs when changed
#666 CI: Move test plugin packages from pyproject.toml to pytest fixtures. Fixes :issue:658
#661:
- Bump minimum version 3.5 -> 3.6
- Drop python 2.7 support
- Modernize syntax (remove future and modesets)
- Update black to 21.6b0