Fix py3 setup

See #17 (closed) for more details, this allows me to install again under both Python 3.5.3 on my Debian laptop and Python 3.6.8 on CIT.

@GregAshton I'm not really fluent in packaging and all changes are ad-hoc hacks, so please have a careful look whether these are all reasonable.

setup.py

@@ -15,7 +15,7 @@ setup(
     author_email="gregory.ashton@ligo.org",
     description="python wrappers for lalpulsar F-statistic code",
     long_description=long_description,
-    long_description_content_type='text/markdown',
+    long_description_content_type="text/markdown",
     packages=find_packages(),
     include_package_data=True,
     package_data={