Add support for Python 3.10

[thisac]

Context: Python 3.10 is supported by all dependencies.

Description of the Change:

• Bumps Python support up to 3.10.
• Bumps versions in the requirements.txt.
• Removes appdirs as a requirement, since it's no longer used.

Benefits: Python 3.10!

Possible Drawbacks: None

Related GitHub Issues: None

[codecov[bot]]

Codecov Report

Merging #695 (a450e55) into master (e6fa90f) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #695   +/-   ##
=======================================
  Coverage   98.69%   98.69%           
=======================================
  Files          75       75           
  Lines        9212     9212           
=======================================
  Hits         9092     9092           
  Misses        120      120           

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