General upgrade

[crusaderky]

• Bumped minimum version of all dependencies
• Added support for Python 3.8 and 3.9
• Removed backports
• Migrated to github workflows for CI
• Run all code linters through pre-commit
• Use setuptools-scm for versioning
• Moved the whole contents of setup.py to setup.cfg

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@04125ac). Click here to learn what that means. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #20   +/-   ##
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  Coverage          ?   94.40%           
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  Files             ?       18           
  Lines             ?      912           
  Branches          ?        0           
=========================================
  Hits              ?      861           
  Misses            ?       51           
  Partials          ?        0           

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