Remove deprecated np.object

[jklaise]

Resolves #888.

[codecov]

Codecov Report

Merging #890 (9f06408) into master (3594f71) will not change coverage.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #890   +/-   ##
=======================================
  Coverage   85.32%   85.32%           
=======================================
  Files          74       74           
  Lines        8779     8779           
=======================================
  Hits         7491     7491           
  Misses       1288     1288           

Flag	Coverage Δ
macos-latest-3.10	?
ubuntu-latest-3.10	85.28% <100.00%> (ø)
ubuntu-latest-3.7	85.19% <100.00%> (ø)
ubuntu-latest-3.8	85.12% <100.00%> (-0.10%)	⬇️
ubuntu-latest-3.9	85.12% <100.00%> (-0.10%)	⬇️
windows-latest-3.9	82.96% <100.00%> (ø)

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Impacted Files	Coverage Δ
alibi/explainers/ale.py	98.26% <100.00%> (ø)
alibi/explainers/partial_dependence.py	94.60% <100.00%> (ø)
alibi/explainers/pd_variance.py	98.78% <100.00%> (ø)