setup.cfg: update keywords

Update old keywords to new format:

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

Author: aspiers

setup.cfg

@@ -1,11 +1,11 @@
 [metadata]
 name = git-deps
-summary = automatically detect dependencies between git commits
+description = automatically detect dependencies between git commits
 author = Adam Spiers
-author-email = git@adamspiers.org
+author_email = git@adamspiers.org
 license = GPL-2+
-home-page = https://github.com/aspiers/git-deps
-description-file = README.md
+home_page = https://github.com/aspiers/git-deps
+long_description = file: README.md, USAGE.md, USE-CASES.md
 classifier =
     Development Status :: 4 - Beta
     Environment :: Console