Update Minimal Python version 3.8

[maxfischer2781]

Community review has shown that all environments in which COBalD is deployed support at least Python 3.8. Since Python 3.7 is EOL now too COBalD will drop support for both Python 3.6 and Python 3.7. This will allow us to use more modern dependencies and remove some cruft from using the older standard library.

This is an overview ticket for the planned two stage deprecation process:

• The next regular release of COBalD will bump the minimal Python version to 3.8
  • Any support for compatibility code or dependencies to Python 3.7/3.6 may be dropped starting in this version
• Ongoing cleanup effort to update code/dependencies to remove cruft and new features
  • See What’s New In Python 3.7 and What’s New In Python 3.8 to identify standard library changes
  • Potentially look at dependencies for changes

See also MatterMiners/tardis#273