pybind: Don't encode str on Python 2

pybind: Don't encode str on Python 2

If you attempt to call encode on a non-ASCII string, then a UnicodeDecodeError
will be raised.

Since str on Python 2 is an 8-bit string, it's possible that it's already UTF-8
encoded. As such we should just pass it through to the C API unmodified.

On Python 3 or if the user explicitly uses unicode, then we'll encode it to
UTF-8 for them.

Signed-off-by: David Coles <dcoles@gaikai.com>