Python 3.11 support

[juhoinkinen]

Allow installing Annif on Python 3.11, and run unit tests on it in the CI/CD pipeline.

Omikuji dependency does not work Python 3.11, see #703 (comment). Other dependencies seem to work (i.e. the unit tests pass).

tomli is now a (direct) dependency only on Python <3.11, on Python 3.11 tomllib of standard library is used instead (PEP 690). However, also on Python 3.11 tomli is still installed as it is a dependency of black, coverage, pytest and schemathesis.

Closes #703, includes merge of the branch of #726.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (320af2b) 99.67% compared to head (f928844) 99.67%.
Report is 18 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #727   +/-   ##
=======================================
  Coverage   99.67%   99.67%           
=======================================
  Files          89       89           
  Lines        6380     6400   +20     
=======================================
+ Hits         6359     6379   +20     
  Misses         21       21           

Files Changed	Coverage Δ
annif/backend/dummy.py	100.00% <ø> (ø)
annif/backend/ensemble.py	100.00% <ø> (ø)
annif/backend/nn_ensemble.py	100.00% <ø> (ø)
annif/backend/omikuji.py	97.46% <ø> (-0.13%)	⬇️
annif/backend/svc.py	100.00% <ø> (ø)
annif/backend/yake.py	98.18% <ø> (-0.07%)	⬇️
tests/test_backend_stwfsa.py	100.00% <ø> (ø)
annif/__init__.py	100.00% <100.00%> (ø)
annif/backend/backend.py	100.00% <100.00%> (ø)
annif/backend/mllm.py	100.00% <100.00%> (ø)
... and 5 more

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[osma]

If the unit tests pass on Python 3.11 (except Omikuji, but that is not even installed for 3.11), then I don't see why we should mark the CI tests as experimental. It would be a different situation if some core tests were failing on 3.11 and we couldn't fix them right now.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[osma]

The Python 3.11 parts of this PR look good to me, but since this includes all the commits from PR #726, we need to make sure those are good as well before merging this. I noticed a potential problem with TF/Keras, see #726 (comment)