setup.py: add tyupe of long_description

This suddenly seems to be necessary.

Author: julianoes

setup.py

@@ -116,6 +116,7 @@ def version():
     version=version(),
     description="Python wrapper for MAVSDK",
     long_description=parse_long_description(),
+    long_description_content_type="text/markdown",
     url="https://github.com/mavlink/MAVSDK-Python",
     maintainer="Jonas Vautherin, Julian Oes",
     maintainer_email="[email protected], [email protected]",