Clean up package metadata

The author-email and python-requires options are deprecated by
SetupTools in favor of author_email and setup_requires and trigger
warnings now, so update them. The license_files option (since its
introduction) automatically includes LICENSE and AUTHORS if not
specified, so was always redundant and unnecessary. The warnerrors
option for PBR was for Sphinx integration, which we don't use any
longer, and is ignored by modern PBR versions anyway so clean it up
too.

Change-Id: Ic5d1e17dd4926eea4e5b6fff47295fd8e9eb452d

setup.cfg

@@ -4,7 +4,7 @@ summary = Tool to submit code to Gerrit
 long_description = file: README.rst
 long_description_content_type = text/x-rst; charset=UTF-8
 author = OpenDev Contributors
-author-email = service-discuss@lists.opendev.org
+author_email = service-discuss@lists.opendev.org
 url = http://docs.opendev.org/opendev/git-review
 project_urls =
     Browse Source = https://opendev.org/opendev/git-review
@@ -15,9 +15,6 @@ project_urls =
     Release Notes = https://docs.opendev.org/opendev/git-review/latest/releasenotes.html
 keywords = git gerrit review commandline
 license = Apache License, Version 2.0
-license_files =
-    AUTHORS
-    LICENSE
 classifiers =
     Development Status :: 5 - Production/Stable
     Environment :: Console
@@ -33,7 +30,7 @@ classifiers =
     Programming Language :: Python :: 3.9
     Programming Language :: Python
     Topic :: Utilities
-python-requires = >= 3.5
+python_requires = >= 3.5
 
 [files]
 packages =
@@ -46,4 +43,3 @@ console_scripts =
 [pbr]
 manpages =
     git-review.1
-warnerrors = True