Don't use "is" for comparing strings.

This is bad mojo, it works on CPython by total chance (and if I were so inclined, I could break it).
Merge pull request #51 from alex/patch-2

Don't use "is" for comparing strings.
PyPy

should work now
Merge pull request #52 from alex/patch-1

PyPy
Merge branch 'master' into my-py3