Fix typechecking with matplotlib 3.8.0

[jklaise]

Fixes #966.

Most of these are just type ignores as they seem to appear as red herrings (types already narrowed implicitly from union types). It would take extra dedicated effort to trace through our type logic and the one for matplotlib and possibly implementation changes to fix this up properly.

[codecov]

Codecov Report

Merging #969 (50291aa) into master (54d0c95) will increase coverage by 0.00%.
Report is 1 commits behind head on master.
The diff coverage is 80.95%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #969   +/-   ##
=======================================
  Coverage   85.23%   85.23%           
=======================================
  Files          74       74           
  Lines        8832     8834    +2     
=======================================
+ Hits         7528     7530    +2     
  Misses       1304     1304           

Files	Coverage Δ
alibi/explainers/pd_variance.py	98.37% <100.00%> (ø)
alibi/prototypes/protoselect.py	93.36% <0.00%> (ø)
alibi/utils/visualization.py	48.10% <62.50%> (ø)

... and 1 file with indirect coverage changes