Previously the code could only handle single constraints when expressing
dependencies in __pkginfo__ as in:

  "blog": ">= 1.1.0",

This improves on the previous situation by handling cases where
multiple constraints are stated as in:

  "blog": ">= 1.1.0, < 1.2.0",

Since we have been switched (or have switched) to using requirements.txt
and pip for installing packages, maybe this whole check could be removed
entirely.