Feat/set up flake8

[tomasvanpottelbergh]

Fixes #718.

• Sets up flake8 to be compatible with Black
• Enforces flake8 in CI
• Add flake8 pre-commit hook

TODO:

• Add .git-blame-ignore-revs file for commit applying black to codebase (see Fix/flake8 issues #748)
• Add more pre-commit documentation

[codecov-commenter]

Codecov Report

Merging #749 (3daf285) into master (96125f8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #749   +/-   ##
=======================================
  Coverage   90.79%   90.79%           
=======================================
  Files          67       67           
  Lines        6734     6734           
=======================================
  Hits         6114     6114           
  Misses        620      620           

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