> But such a test doesn't belong in the test suite.

py2exe uses the "modulefinder" module to find dependencies.

I tried this command::

    ./python -m modulefinder Lib\email\message.py

And among the output, there is::

    ....
    ? email.Generator imported from __main__, email.message
    ? email.Iterators imported from __main__, email.message
    ....

It surely can be turned into a unit test. For example::

    import email.message
    filename = email.message.__file__
    if filename.endswith(".pyc"):
        filename = filename[:-1]

    import modulefinder
    mf=modulefinder.ModuleFinder()
    mf.run_script(filename)
    assert "email.Generator" not in mf.any_missing()