Drop Python 2 support & enable GitHub Actions

[greyli]

• Drop Python 2 and 3.5 support.
• Enable CI for Python 3.6-3.10.
• Remove the copyright headers.
• Update project links.
• Update example applications to support Flask-SQLAlchemy 3.x.

fixes #60
fixes #61

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (master@33439ff). Click here to learn what that means.
Patch has no changes to coverable lines.

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Additional details and impacted files

@@            Coverage Diff            @@
##             master      #62   +/-   ##
=========================================
  Coverage          ?   82.64%           
=========================================
  Files             ?        3           
  Lines             ?      121           
  Branches          ?        0           
=========================================
  Hits              ?      100           
  Misses            ?       21           
  Partials          ?        0           

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