Use pytest-custom_exit_code to not flag empty notebook test suite as …

[jklaise]

…a CI failure

Resolves #647. @ascillitoe if happy with implementation I'll open an identical PR for alibi-detect.

[ascillitoe]

LGTM!

[codecov]

Codecov Report

Merging #728 (9aec082) into master (f66d5ba) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #728   +/-   ##
=======================================
  Coverage   81.06%   81.06%           
=======================================
  Files         105      105           
  Lines       11794    11794           
=======================================
  Hits         9561     9561           
  Misses       2233     2233           

Impacted Files	Coverage Δ
alibi/explainers/anchors/anchor_base.py	92.70% <0.00%> (ø)
alibi/explainers/anchors/anchor_text.py	94.44% <0.00%> (ø)
alibi/explainers/anchors/anchor_image.py	93.12% <0.00%> (ø)
alibi/explainers/anchors/anchor_tabular.py	90.17% <0.00%> (ø)