Python: Add a long_description to appease twine.

Unclear whether this was intentional on the python tooling side, but
twine (the tool that uploads packages to PyPI) now fails if the
long_description is missing. See pypa/twine#960
for details.

setup.py

@@ -309,6 +309,7 @@ def get_basesources(name, osname):
 setup (name = 'psychtoolbox',
        version = version,
        description = 'Pieces of Psychtoolbox-3 ported to CPython.',
+       long_description = 'Pieces of Psychtoolbox-3 ported to CPython.',
        author = 'Mario Kleiner',
        author_email = 'mario.kleiner.de@gmail.com',
        url = 'http://psychtoolbox.org',