setup.py: Actually pass long_description_content_type

It of course also needs to be passed to the setup function... :]

Otherwise we get:
    Checking dist/btrfs-14.1.tar.gz: PASSED with warnings
    WARNING  `long_description_content_type` missing. defaulting to `text/x-rst`.

Author: knorrie

setup.py

@@ -39,6 +39,7 @@ def get_version():
     version=version,
     description='Python module to interact programmatically with an online btrfs file system',
     long_description=long_description,
+    long_description_content_type=long_description_content_type,
     author='Hans van Kranenburg',
     author_email='[email protected]',
     url='https://github.com/knorrie/python-btrfs',