fix rtd build error and upgrade to python 3.9

[wang-boyu]

Fix build error in readthedocs:

Error

Problem in your project's configuration. Invalid configuration option "build.os": build not found

Also upgrade to Python 3.9 following Mesa.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a128776) 75.90% compared to head (c479429) 75.90%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #176   +/-   ##
=======================================
  Coverage   75.90%   75.90%           
=======================================
  Files          10       10           
  Lines         660      660           
  Branches      138      138           
=======================================
  Hits          501      501           
  Misses        139      139           
  Partials       20       20           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.