Remove pytest-pep8

[yuxuanzhuang]

Fixes #139 Fixes #143

Changes made in this Pull Request:

• remove pep8 check in pytest
• pin to matplotlib 3.2

PR Checklist

• [ ] Tests?
• [ ] Docs?
• [ ] CHANGELOG updated?
• [x] Issue raised/referenced?

[codecov[bot]]

Codecov Report

Merging #142 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #142   +/-   ##
=======================================
  Coverage   97.93%   97.93%           
=======================================
  Files          12       12           
  Lines         775      775           
  Branches       91       91           
=======================================
  Hits          759      759           
  Misses         10       10           
  Partials        6        6           

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[orbeckst]

Thanks!