ci: Add Python 3.10

[nijel]

Proposed changes

Add Python 3.10 to the test matrix.

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[nijel]

@omab It would be great to add Python 3.10 to required tests in branch protection settings (I can't change that).

[omab]

@nijel, done

[nijel]

This now includes #653 by @KOliver94 to make it mergeable (Python 3.10 is already enforced, but the tests fail without #653).

[codecov]

Codecov Report

Merging #647 (a0098c1) into master (1ea27e8) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #647      +/-   ##
==========================================
- Coverage   76.92%   76.90%   -0.02%     
==========================================
  Files         317      317              
  Lines        9630     9635       +5     
  Branches      962     1033      +71     
==========================================
+ Hits         7408     7410       +2     
- Misses       2071     2074       +3     
  Partials      151      151              

Flag	Coverage Δ
unittests	76.90% <ø> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
social_core/backends/coding.py	0.00% <0.00%> (ø)
social_core/backends/azuread_tenant.py	0.00% <0.00%> (ø)
social_core/backends/azuread.py	74.00% <0.00%> (+1.08%)	⬆️

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