This does a temporary sdist build and extracts the result in order to
get the (singular) '>=' lower bound.

If the expectations of the script are violated, an error is raised.

The approach is to be "slow but robust" here. We could simply parse
`setup.cfg`, but that makes the toolchain sensitive to changes in
packaging tooling. The PKG-INFO data should be reliable in all cases.

This check requires a YAML parser to pull the github workflow data
and crawl to the correct node.
Once there, the check will assert that the version in the CI build
*exactly* matches the lower bound in `Requires-Python`.

This leverages the `get_python_requires.py` script to extract the
requirements dir.

Allow passing a cache directory explicitly to
`get_python_requires.py`. Subsequently, the wrapping script,
`ensure_min_python_is_tested.py` can insist on an env var which
provides the cache dir path.
Finally, `tox.ini` gets a final tweak to set this value via setenv to
the `envdir` (the virtualenv dir which also contains the tmp and log
dirs).

The choice of an env var over parsing an argument is merely that it is
marginally simpler to read an env var and fail on its absence than it
is to do the same with an argument. (At the very least, *any* argument
parsing opens the question of whether or not argparse should be used.)

mddj is a public packaged version of the same functionality (in Alpha).

Pulling it out of this repo lets us rely on it without having to
carry the maintenance burden in globus-sdk of all of the relevant logic.

This uses a new feature of `mddj` to read the tox factor list and
confirm that the minimum version declared in our requires-python data
is also synchronized with our tox.ini