build: Add long_description_content_type to address warning from twin…

…e check

setup.cfg

@@ -7,6 +7,7 @@ license = BSD
 description = Tools for data standards that use JSON Schema and CSV codelists
 url = https://github.com/open-contracting/jscc
 long_description = file: README.rst
+long_description_content_type = text/x-rst
 classifiers =
     License :: OSI Approved :: BSD License
     Operating System :: OS Independent