setup: Use summary for package summary

Otherwise, the long_description file isn't used.

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

setup.cfg

@@ -1,6 +1,6 @@
 [metadata]
 name = git-pw
-description = Git-Patchwork integration tool
+summary = Git-Patchwork integration tool
 long_description = file: README.rst
 long_description_content_type = text/x-rst; charset=UTF-8
 license = MIT License