Ensure that twine check returns no warnings

Set the long_description_content_type to text/x-rst

Signed-off-by: Philippe Ombredanne <pombredanne@nexb.com>

Author: pombredanne

setup.cfg

@@ -15,6 +15,7 @@ license = Apache-2.0 AND BSD-3-Clause AND MIT
 # description must be on ONE line https://github.com/pypa/setuptools/issues/1390
 description = A mostly universal library to parse and compare software package versions and version ranges. A companion to Package URLs.
 long_description = file:README.rst
+long_description_content_type = text/x-rst
 url = https://github.com/nexB/univers
 classifiers =
     Development Status :: 5 - Production/Stable