MAINT Add pre-commit, simplify flake8 setup

[rth]

• Simplify flake8 usage by putting all relevant config in a setup.cfg file
• add a documentation section on code style under contributing
• add a minimal pre-commit to make it more convenient to run linters before each commit (use is optional)

[codecov[bot]]

Codecov Report

Merging #267 (3605be1) into master (e97a272) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #267   +/-   ##
=======================================
  Coverage   82.13%   82.13%           
=======================================
  Files         133      133           
  Lines        4938     4938           
=======================================
  Hits         4056     4056           
  Misses        882      882           

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