Pass long_description_content_type into setup()

Author: cderici

setup.py

@@ -39,6 +39,7 @@
     maintainer_email='juju@lists.ubuntu.com',
     description=('Python library for Juju'),
     long_description=long_description,
+    long_description_content_type=long_description_content_type,
     url='https://github.com/juju/python-libjuju',
     license='Apache 2',
     classifiers=[