Add flake8-pytest-style

[fealho]

As part of #80, adds flake8-pytest-style and adapt the current code to pass this lint.

[codecov-commenter]

Codecov Report

Merging #89 (967baf4) into master (d38e1ff) will decrease coverage by 22.76%.
The diff coverage is 51.03%.

@@             Coverage Diff             @@
##           master      #89       +/-   ##
===========================================
- Coverage   73.93%   51.16%   -22.77%     
===========================================
  Files          21       50       +29     
  Lines         468     1503     +1035     
===========================================
+ Hits          346      769      +423     
- Misses        122      734      +612     

Impacted Files	Coverage Δ
sdmetrics/single_table/efficacy/mlefficacy.py	0.00% <0.00%> (ø)
sdmetrics/timeseries/ml_scorers.py	19.04% <19.04%> (ø)
sdmetrics/single_table/gaussian_mixture.py	26.47% <26.47%> (ø)
sdmetrics/__init__.py	42.85% <29.41%> (-57.15%)	⬇️
...cs/single_table/privacy/radius_nearest_neighbor.py	31.25% <31.25%> (ø)
sdmetrics/single_table/privacy/base.py	32.53% <32.53%> (ø)
sdmetrics/single_table/base.py	35.89% <35.89%> (ø)
sdmetrics/single_table/privacy/loss.py	39.13% <39.13%> (ø)
sdmetrics/single_table/efficacy/base.py	40.00% <40.00%> (ø)
sdmetrics/timeseries/efficacy/base.py	41.30% <41.30%> (ø)
... and 40 more

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