Centralize text common to code and documentation (e.g., required python version numbers)

[rountree]

We'd like to have the benchpark binary be able to check the version of python in use against the minimum version, mention the minimum version in the documentation, and both tied to a common source so there's only a single file that needs to get updated.

rst has a .. literalinclude:: tag that should fit the bill. Perhaps have a file ground_truth/min_python_version.txt that is included via literalinclude in the docs and (with a bit more work) parsed by python in the benchpark script.

This avoids any CI silliness.