pre-commit: Update flake8 hook configuration

flake8 hook has been removed from https://github.com/pre-commit/pre-commit-hooks so now use the one from https://gitlab.com/pycqa/flake8

See error output below after executing pre-commit autoupdate:

pre-commit run --all-files
[INFO] Installing environment for https://github.com/pre-commit/pre-commit-hooks.
[INFO] Once installed this environment will be reused.
[INFO] This may take a few minutes...
[INFO] Installing environment for https://github.com/codespell-project/codespell.
[INFO] Once installed this environment will be reused.
[INFO] This may take a few minutes...
[INFO] Installing environment for https://github.com/python/black.
[INFO] Once installed this environment will be reused.
[INFO] This may take a few minutes...
Trim Trailing Whitespace.................................................Passed
Flake8 (removed).........................................................Failed
- hook id: flake8
- exit code: 1

`flake8` has been removed -- use `flake8` from https://gitlab.com/pycqa/flake8

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Migrated from D3971 (view on Phabricator)