Fix setup.cfg keywords

author-email, home-page are aliases, replacing them for their final value

description-file -> long_description = file:

requires-python is old value and does not work, python-requires
is right metadata.

'python-requires' is introduced from PEP440 and requires-python was old value
in PEP345.
- https://packaging.python.org/guides/distributing-packages-using-setuptools/#python-requires
- https://www.python.org/dev/peps/pep-0345/#requires-python

Change-Id: I2d2cd5d660ef937bfe8445bae7cf6b279eb80b0f

setup.cfg

@@ -1,12 +1,11 @@
 [metadata]
 name = barbican_tempest_plugin
 summary = OpenStack barbican tempest tests.
-description-file =
-    README.rst
+long_description = file: README.rst
 author = OpenStack
-author-email = openstack-dev@lists.openstack.org
-home-page = http://www.openstack.org/
-requires-python = >=3.6
+author_email = openstack-dev@lists.openstack.org
+url = http://www.openstack.org/
+python_requires = >=3.6
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology