Fix readthedocs Webhook failure

[trundev]

This is to fix the annoying failure when creating PRs.

The "readthedocs" server, seems to require a build.os parameters, as answered here.

I changed the Python version to 3.9, just to have a longer live-span (it needed to drop sphinx version limit).
Migration to a later python version need more work, not sure if this is really needed.

In addition, the now obsolete numba~=0.48.0 note can be removed from here: https://clifford.readthedocs.io/en/latest/installation.html#pypi

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.15%. Comparing base (568087f) to head (af82858).
Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #440      +/-   ##
==========================================
+ Coverage   71.13%   71.15%   +0.02%     
==========================================
  Files          74       74              
  Lines        9437     9437              
  Branches     1211     1211              
==========================================
+ Hits         6713     6715       +2     
+ Misses       2551     2548       -3     
- Partials      173      174       +1     

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