Update pre-commit with flake8

[CasperWA]

Add flake8 to pre-commit and removing the lint job from the CI workflow, since it is now included in the pre-commit job.

Use caching for pre-commit in CI.

[codecov]

Codecov Report

Merging #186 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #186   +/-   ##
========================================
  Coverage    34.12%   34.12%           
========================================
  Files           18       18           
  Lines         2113     2113           
========================================
  Hits           721      721           
  Misses        1392     1392           

Flag	Coverage Δ
#optimade-client	34.12% <ø> (ø)

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