Ensure we can handle old distutils-only setup.py

[pombredanne]

From #66 (comment)

handle the rarer case where the setup.py is using plain distutils See https://github.com/search?l=Python&q=from+distutils.core+import+setup+filename+filename%3Asetup.py&type=Code for some exotic examples.