Remove flake8 and black Nox sessions.

[trexfeathers]

In line with Iris, this PR proposes relying purely on pre-commit.ci (already enabled on this repo) for linting tasks, doing away with independently managed Nox sessions.

This reduces duplication, and removes the risk of divergence between the Nox sessions and the pre-commit setup (which recently caused this CI failure - different linter versions were insisting on different standards).

Iris' decision-making history:

• Use pre-commit instead of an independent setup: SciTools/iris#4181
• Rely purely on pre-commit.ci without an additional Nox session: SciTools/iris#4503

[codecov[bot]]

Codecov Report

Merging #236 (01bfcdb) into main (dbd1cb7) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #236   +/-   ##
=======================================
  Coverage   99.36%   99.36%           
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  Files          28       28           
  Lines        2829     2829           
=======================================
  Hits         2811     2811           
  Misses         18       18           

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[trexfeathers]

I believe black and flake should now not be needed in the requirements files. They are absent from the Iris requirements (removed with this PR SciTools/iris#4181).

Good catch, thanks

[trexfeathers]

Looks like the pre-commit config is missing the appropriate subcommands to mimic what was happening in the Nox session. Will look later.