Fix twine check warning

warning: `long_description_content_type` missing.

Author: mayeut

setup.cfg

@@ -6,6 +6,7 @@ author = Robert T. McGibbon
 author-email = [email protected]
 summary = Cross-distribution Linux wheels
 description-file = README.rst
+long_description_content_type = text/x-rst
 python_requires = >=3.6
 classifier =
         Development Status :: 4 - Beta