Python: Add a long_description to appease Python's publishing tools.

It seems this is unintentional and may be fixed one day, but
twine (the tool that uploads packages to PyPI) now fails if the
long_description is not set. See pypa/twine#960
for context.

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 = '[email protected]',
        url = 'http://psychtoolbox.org',