Missing warning about cymunk not being available

[KeyWeeUsr]

cymunk module setup.py

Traceback (most recent call last):
  File "setup.py", line 101, in <module>
    cymunk_dirname = loader.path if hasattr(loader, 'path') else loader.filename
AttributeError: 'NoneType' object has no attribute 'filename'

I think pkgutil returns None when it can't find the required package, therefore it might be a good idea to ping the user about installing it, or add it to requirements (but there's no cymunk on pypi, so that probably nah).