Automatically bump versions

[caspervdw]

(below is also in the README.md)

Managing dependencies

clean-python has all of its dependencies pinned, because in that way the automated tests run
in the same environment that the application that uses clean-python. The requirements are automatically
updated each week by the GH Actions script located in .github/workflows/bump.yml.

This works as follows:

• Requirements are specified in requirements/*.in files. Mostly, they do not have version specifies.
• To ensure that all optional requirements are consistent with one another, we first generate
  a single requirement file all.txt from all *.in files together. We use pip-tools for that
  (see https://github.com/jazzband/pip-tools).
• Then for each *.in file, a requirements file is generated. This requirement file is constrained to
  the consistent set generated in the previous step using -c all.txt.
  For instance the fastapi.txt is generated from fastapi.in.
• The pyproject.toml refers to requirement files from its dependencies and optional-dependencies
  section. Each optional dependency has its own requirement file.

The result of this is that you can install clean-python with any combination of optional dependencies.

[caspervdw]

Superseded by #101