[BREAKING] Deprecate support for Python 3.6

[madpah]

Discussed in #216

Python 3.6 is reached its end of life as of December 2021.

This feature will drop support for Python 3.6.

FYI: @jkowalleck

• Update pyproject.toml
  • classifiers
  • dependnecy minimum-versions
  • and other such data
• Remove Python 3.6 tests from CI
• remove 3.6 from CI/CT (github workflows)
• adjust requirements.lowest.txt to versions that at least support python3.6
• migrate code to the features of py37

TARGET BRANCH: dev/3.0.0

[madpah]

Included in 4.0.0 and contributed by @hakandilek - many thanks!