chore: Drop support of py36 and py37

[zhongjiajie]

Because both of them are EOL, which you
can see in https://devguide.python.org/versions/

Brief Summary of The Change

Pull Request checklist

I confirm that the following checklist has been completed.

• Add/Change test cases for the changes.
• Add/Change the related documentation, should also change docs/source/config.rst when you change file default_config.yaml.
• (Optional) Add your change to UPDATING.md when it is an incompatible change.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6e5c6e9) 90.83% compared to head (7805cf8) 90.83%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #126   +/-   ##
=======================================
  Coverage   90.83%   90.83%           
=======================================
  Files          64       64           
  Lines        2345     2345           
=======================================
  Hits         2130     2130           
  Misses        215      215           

Flag	Coverage Δ
unittests	90.83% <ø> (ø)

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