Update pytest and clean up tests a bit

[devin-petersohn]

• Contributes to CI Updates and Improvements #853

What do these changes do?

Related issue number

• passes flake8 modin
• passes black --check modin
• tests added and passing

[codecov]

Codecov Report

Merging #903 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #903      +/-   ##
==========================================
+ Coverage   87.71%   87.71%   +<.01%     
==========================================
  Files          54       54              
  Lines        5737     5739       +2     
==========================================
+ Hits         5032     5034       +2     
  Misses        705      705

Impacted Files	Coverage Δ
modin/pandas/dataframe.py	89.06% <100%> (+0.02%)	⬆️

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