Add support for pypy

Fortunately pypy provides support for a lot of the CPython API, so the
changes are minimal.

The most important changes are:

- constructors always get a keyword argument dictionary, even if no
  keyword arguments are passed

- trying to assign to a read-only attribute raises TypeError instead of
  AttributeError

Apart from that, pypy does not provide MAXPATHLEN. There is a hack in
place currently, but there is only place that's using that macro, and
there shouldn't be a need for it much longer.

This fixes #209.
Merge remote-tracking branch 'carlos/pypy'