MAINT cleanup numpy warnings related to np.matrix in tests

[ogrisel]

There still remain a few occurrences of np.matrix for the pickle compat tests. Cleaning those would require regenerating the stored pickled files with old Python & joblib versions.

We can do that in a later PR ;)

[codecov]

Codecov Report

Base: 93.96% // Head: 93.96% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (8c8c33a) compared to base (cea26ff).
Patch coverage: 80.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1340      +/-   ##
==========================================
- Coverage   93.96%   93.96%   -0.01%     
==========================================
  Files          52       52              
  Lines        7294     7292       -2     
==========================================
- Hits         6854     6852       -2     
  Misses        440      440              

Impacted Files	Coverage Δ
joblib/test/test_numpy_pickle.py	94.23% <80.00%> (+0.13%)	⬆️
joblib/memory.py	95.25% <0.00%> (-0.27%)	⬇️

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