python>=3.8 func_inspect: allow POSITIONAL_ONLY parameters

[ptrba]

This PR enables correct caching for functions with positional-only arguments. This breaks compatibility with python3.7. As soon as python3.7 support is replaced by python>=3.8, this can be merged.

Positional only parameters (added in python3.8) are not hashed. This is due to filter_args which handles POSITIONAL_OR_KEYWORD parameters but ignores POSITIONAL_ONLY parameters. Since hashing uses a name-value tuple list, it is possible to treat POSITIONAL_ONLY equivalently to POSITIONAL_OR_KEYWORD parameters.

[codecov]

Codecov Report

Merging #1313 (ae0b915) into master (1fdf308) will decrease coverage by 0.63%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #1313      +/-   ##
==========================================
- Coverage   93.90%   93.27%   -0.64%     
==========================================
  Files          50       50              
  Lines        7270     7272       +2     
==========================================
- Hits         6827     6783      -44     
- Misses        443      489      +46     

Impacted Files	Coverage Δ
joblib/test/test_func_inspect.py	90.00% <50.00%> (-0.63%)	⬇️
joblib/func_inspect.py	92.34% <100.00%> (ø)
joblib/test/common.py	67.24% <0.00%> (-10.35%)	⬇️
joblib/pool.py	85.36% <0.00%> (-3.26%)	⬇️
joblib/hashing.py	88.59% <0.00%> (-2.64%)	⬇️
joblib/numpy_pickle_utils.py	91.01% <0.00%> (-2.25%)	⬇️
joblib/backports.py	67.67% <0.00%> (-2.03%)	⬇️
joblib/numpy_pickle.py	97.43% <0.00%> (-1.71%)	⬇️
joblib/parallel.py	94.96% <0.00%> (-1.60%)	⬇️
joblib/logger.py	85.52% <0.00%> (-1.32%)	⬇️
... and 5 more

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