tests/testutils/python_repo.py: Use subprocess to run sdist

The current approach using setuptools.sandbox.run_setup() was
causing a spurious (but highly frequent) failure where setuptools
gets mixed up with it's manipulation of sys.modules and hits a
RuntimeError as a dictionary changes size while being iterated over.

For instance: https://gitlab.com/BuildStream/buildstream/-/jobs/147967307

Since this already happens in an isolated virtual environment created
by tox, we should not need additional sandboxing here from setuptools,
and launching this as a subprocess will be safer.