Fix tests

[oshillou]

Due to the recent changes of numpy, I had to change the testing procedures:

• Abandonned testing for Python 3.8 as this does not comply with a sufficiently up-to-date version of numpy, that other versions uses, and this poses problem when compiling Cython code.

I fixed also the data validation procedure by adapting to newer function names in scikit-learn, i.e. validate_data instead of self._validate_data in each fit and prediction method.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.41%. Comparing base (48816d4) to head (a149d31).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #10   +/-   ##
=======================================
  Coverage   96.40%   96.41%           
=======================================
  Files          31       31           
  Lines        1838     1840    +2     
=======================================
+ Hits         1772     1774    +2     
  Misses         66       66           

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