LanguageModelSampler punctuation fix

[RobertSamoilescu]

This PR fixes the punctuation sampling reported in #582 by re-writing the is_punctuation method and the corresponding test_lm_sample_punctuation test.

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[jklaise]

Line #2.    os.environ['TF_CPP_MIN_LOG_LEVEL'] = '3'

Should we remove this from the notebook? Or if it's related to surpressing transformers output then leave a short comment for the line.

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[jklaise]

LGTM!

[codecov]

Codecov Report

Merging #585 (0cb6728) into master (529943d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #585   +/-   ##
=======================================
  Coverage   82.05%   82.06%           
=======================================
  Files          77       77           
  Lines       10517    10520    +3     
=======================================
+ Hits         8630     8633    +3     
  Misses       1887     1887           

Impacted Files	Coverage Δ
alibi/explainers/anchor_text.py	87.05% <100.00%> (+0.03%)	⬆️
alibi/explainers/tests/test_anchor_text.py	93.15% <100.00%> (ø)
alibi/utils/lang_model.py	96.69% <100.00%> (+0.05%)	⬆️
alibi/version.py	100.00% <100.00%> (ø)

[ascillitoe]

Nice @RobertSamoilescu. I've fixed the other CI issue we were having so hopefully CI will pass now.