Python 3 & getcwdu

We were using the `os.getcwdu()` function in several places. That doesn't exist on Python 3, but the path.py module was masking that by setting `os.getcwdu = os.getcwd`. As described in #4462, however, that didn't always work.

This adds a reference in `py3compat` which points to getcwd on Python 3 and getcwdu in Python 2.