Bug report

1. build from the main branch
2. cd $src/Doc
3. make clean-venv
4. make venv PYTHON=$build/python
5. make doctest PYTHON=$build/python

that is a correct way to run the doctests... but if you forget the PYTHON= setting to point at a just-built-from-tree Python, it'll happily use the system installed python interpreter and give you loads of version mismatch missing feature or syntax related failures.

We should make this more robust so that make doctest refuses to run under a python not built from your source tree.