Fixes some pylint errors and re-enables CI tests for them.

[mastersplinter]

Per #127 which introduced pylinting and #129, this PR:

• makes the code comply with pylint by fixing errors ticked in Remove pylint disabled exceptions #129
• re-enables pylinting for those specific exceptions.

[codecov-io]

Codecov Report

Merging #135 into master will increase coverage by <.01%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master     #135      +/-   ##
==========================================
+ Coverage   95.91%   95.92%   +<.01%     
==========================================
  Files           8        8              
  Lines         441      442       +1     
==========================================
+ Hits          423      424       +1     
  Misses         18       18

Impacted Files	Coverage Δ
pandera/dtypes.py	100% <ø> (ø)	⬆️
pandera/schemas.py	100% <100%> (ø)	⬆️
pandera/hypotheses.py	88% <83.33%> (ø)	⬆️
pandera/checks.py	95.69% <84.61%> (+0.04%)	⬆️

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