setup.py: Set long_description_content_type

It defaults to ReStructured Text, but commit eb0b1aa changed the
README to Markdown.

See https://packaging.python.org/en/latest/guides/making-a-pypi-friendly-readme/

Fixes martinpitt#106

Author: martinpitt

setup.py

@@ -14,6 +14,7 @@
     version=version,
     description='Mock D-Bus objects',
     long_description=readme,
+    long_description_content_type='text/markdown',
     author='Martin Pitt',
     author_email='martin.pitt@ubuntu.com',
     url='https://github.com/martinpitt/python-dbusmock',