Convert the path via "y" in Python 3, and rename the format alias to
cstr_argf to more accurately reflect what it means, i.e. a sequence of
non-null bytes. If "y" ends up being sufficient it's simpler (and
more efficient?) than a Py_buffer conversion, though it doesn't
support as many argument types.
Signed-off-by: Rob Browning <rlb@defaultvalue.org>