Now that we have ensurepip, is there any reason to not have the Doc/ Makefile create a venv for building the docs instead of requiring people to install sphinx into either their global Python interpreter or some venv outside of their checkout? Basically it would be like going back to the old Makefile of checking out the code but instead do a better isolation job and let pip manage fetching everything, updating the projects, etc.
I'm going to at least update the Makefile to have a ``make venv`` that will do the steps necessary to create a venv and then check out the necessary tools. It should also be set up so that running the command with a pre-existing venv will update it. This should all lead to commands like ``make html PYTHON=venv/bin/python3`` to work.
Attached is a patch against Doc/Makefile to add a venv command to create a venv that can be used to build the documentation. Georg, can you give me an LGTM so I can commit this?
