... containing all top-level (*.in) and concrete requirements.txt, the
latter parametrized by python version.

We have a bunch of dependencies that are only needed on specific python
versions (e.g. backports like dataclasses, importlib_resources, etc).
We use conditional environment markers for those in the top-level
requirements.in files.
pip-compile does not yet support generating a combined requirements.txt
file where differences across python versions/platforms/archs are
reconciled using environmet markers:
jazzband/pip-tools#826

They currently recommend running pip-compile on each targeted python
environment generating as many concrete requirements.txt files
jazzband/pip-tools#651

So this is what I have done here.

…irements.in files

pip-compile setup.py doesn't support environment markers ('; python_version < 3.7'), whereas it does when compiling from requirements.in files.
So we read top-level dependencies from external *.in files and stick
them in the setup.py install_requires and extras_require keywords.

The 'dev' extra is new. One can now do `pip install -e .[dev]` and have
all the development requirements installed alongside the current module.
This is handy for developers bootstrapping a new venv to work on.
It's a common practice among python developers.

This is handy when any top-level deps change. The shell script is just a shorthand for the longish tox command

tox -e py39-dev will install in editable mode (for when one is in a rush).
One can also use this as a way to quickly bootstrap a development virtual environment.
E.g. after running the above command, a new venv is created in .tox/py39-dev with all
the pinned requirements installed and the current module installed in editable mode,
ready for editing source files and running directly the pytest command therein

The generic invocation 'tox -e py' does not work any more, because we now install dependencies using python-version-specific requirements.txt files and we need to know the exact python major.minor version.
Thus we set TOXENV environment variable such that invoking 'tox'
without additional -e options will run only the specified python
version.

Using tox -l command to list them, so we have a single source of truth for what are the versions that we support. And we only need to change one file when, e.g., python3.10 is out.

otherwise pip-compile will write full absolute paths in the generated requirements.txt..