setup: Set content type of README, avoid aliases

Set the context type of the README and replace aliases with their
main key (based on [1]).

[1] https://setuptools.pypa.io/en/latest/userguide/declarative_config.html#metadata

Signed-off-by: Stephen Finucane <stephen@that.guru>

Author: stephenfin

setup.cfg

@@ -1,7 +1,8 @@
 [metadata]
 name = git-pw
-summary = Git-Patchwork integration tool
-description_file = README.rst
+description = Git-Patchwork integration tool
+long_description = file: README.rst
+long_description_content_type = text/x-rst; charset=UTF-8
 license = MIT License
 license_file = LICENSE
 classifiers =