build!: move requirements back to setup.cfg

[redeboer]

With the release of pip-tools v6.1.0, pip-compile now correctly handles extras_require. This allows for a big clean-up of the set-up introduced by ComPWA/expertsystem#412.

Advantages:

• Easier for the user and developer! Just run:
  pip install -e .[dev]
  to get started developing,
  pip install -e .[dev] -U
  to try out upgrades, and
  pip install -c .constraints/py3.8.txt -e .[dev] -U
  in case you exactly want to reproduce the dependencies for some commit.
• The upgrade jobs become significantly faster: bit over 1min versus almost 3min
• All requirements, their version domains, and the dependencies between each of the requirement sections can be nicely overseen and maintained within setup.cfg
• Git diffs for pull requests that affect dependencies become easier to oversee
• Less complexity and fewer changes per commit