Add support for Path-like objects in Python 3.6+

Add support for Path-like objects in Python 3.6+

This is done by switching to PyUnicode_FSConverter (3.1+), which
supports it. The convert_obj() function is now much simpler, which is
a bonus.

Expand tests to check behaviour with path objects, and skip those on
Python 3.6.

Closes #20.