Make CI enforce pre-commit clean code (#1096)

* pre-commit: Enforce pre-commit clean code via CI

* ci.yml: Resolve duplication of isort and yapf

By now pre-commit is already running these.

* ci.yml: Move flake8 to pre-commit

* README.rst: Add CI status badge for workflow "Run pre-commit"

* README.rst: Fix link for CI workflow "YAPF" (ci.yml)
4 files changed