bpo-25094: Fix test_tools.test_sundry() on Windows (GH-8406)

bpo-25094: Fix test_tools.test_sundry() on Windows (GH-8406)

When Python is installed on Windows, python -m test test_tools failed
because it tried to run Tools\scripts\2to3.py which requires an
argument. Skip this script. On other platforms or on Windows but when
run from source code (not installed), the script is called "2to3"
instead of "2to.py" and so was already skipped.

Modify also the unit test to unload all modules which have been
loaded by the test.
(cherry picked from commit 752d4b7531093c55d6f0a5846748f981d79b29d3)

Co-authored-by: Victor Stinner <vstinner@redhat.com>