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.

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

See merge request !1090

Chandan Singh authored Jan 18, 2019 and Chandan Singh committed Jan 18, 2019

The `import` element is not really a build element. The main purpose of
the `BuildElement` class is to allow users to run `build-commands`,
`install-commands` etc. But, `import` does not run such commands.
Moreover, we already override all the methods provided by
`BuildElement`. So it only makes it confusing to have it derived from
`BuildElement` class when it is not a build element.

So, derive it from the base `Element` class instead.