Add pre-commit hooks

[PiispaH]

Now both isort and black are included in the pre-commit hooks.

Fixes spine-tools/Spine-Toolbox#2897

Checklist before merging

• Documentation (also in Toolbox repo) is up-to-date
• Release notes in Toolbox repo have been updated
• Unit tests have been added/updated accordingly
• Code has been formatted by black & isort
• Unit tests pass

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.69%. Comparing base (8da9d87) to head (58c1629).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #147   +/-   ##
=======================================
  Coverage   62.69%   62.69%           
=======================================
  Files          40       40           
  Lines        3855     3855           
  Branches      767      767           
=======================================
  Hits         2417     2417           
  Misses       1290     1290           
  Partials      148      148           

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