Add Pre-Commit Config

[fcollonval]

Adds support for pre-commit following the example from jupyter_client.

[codecov-commenter]

Codecov Report

Merging #575 (278844a) into master (1444737) will increase coverage by 0.18%.
The diff coverage is 82.59%.

@@            Coverage Diff             @@
##           master     #575      +/-   ##
==========================================
+ Coverage   76.93%   77.12%   +0.18%     
==========================================
  Files         109      109              
  Lines        9951    10115     +164     
  Branches     1079     1089      +10     
==========================================
+ Hits         7656     7801     +145     
- Misses       1912     1929      +17     
- Partials      383      385       +2     

Impacted Files	Coverage Δ
jupyter_server/auth/logout.py	44.44% <0.00%> (ø)
jupyter_server/prometheus/log_functions.py	100.00% <ø> (ø)
jupyter_server/serverapp.py	65.53% <ø> (-0.38%)	⬇️
..._server/tests/extension/mockextensions/__init__.py	100.00% <ø> (ø)
...ter_server/tests/extension/mockextensions/mock3.py	100.00% <ø> (ø)
jupyter_server/tests/test_version.py	100.00% <ø> (ø)
jupyter_server/tests/test_view.py	96.96% <ø> (ø)
jupyter_server/tests/unix_sockets/conftest.py	94.44% <ø> (ø)
jupyter_server/tests/unix_sockets/test_api.py	96.96% <ø> (-0.33%)	⬇️
...r/tests/unix_sockets/test_serverapp_integration.py	21.83% <ø> (-0.89%)	⬇️
... and 117 more

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[blink1073]

Let's merge this, and then rebase #571 which has relevant fixes for CI.

[blink1073]

Thank you!