This could be run with at least three different ways to invoke pytest:
through the shell, through "python -m pytest" and inlined. As inlined
would be the fastest it could be run first (or "--fast" mode).
+The idea is that you can e.g. import modules in a test and afterwards
+sys.modules, sys.meta_path etc would be reverted. It can go further
+then just importing however, e.g. current working direcroty, file
+This would probably be done by marking::
+ @pytest.mark.isolate(importing=True, cwd=True, fds=False)
+With the possibility of doing this globally in an ini-file.