Move developer & CI dependencies into separate requirements file

[mhucka]

What is the task?

The CI workflow in ci.yaml pip-installs at least one Python package (flake8) that is not listed elsewhere nor added by the write-ci-requirements.py script. In addition, requirements.txt loads sphinx, black, and pytest, which are also dev dependencies.

(Optional) Do you have ideas or preferences for the approach?

It would probably be best to create a requirements-dev.txt file, or a file in dev_tools/requirements, and move things like pytest into it, plus add flake8 and anything else loaded by the CI workflows.

How urgent is this for you?

P1 – needed by the next release