Replace old requires-python with python-requires

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: I6a28757f4a0bf5d47214f4ed548cf5c73d302e3f
diff --git a/setup.cfg b/setup.cfg
index 1ac729c..79f30be 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -6,7 +6,7 @@
 author = OpenStack
 author-email = openstack-discuss@lists.openstack.org
 home-page = https://opendev.org/openstack/neutron-tempest-plugin
-requires-python = >=3.6
+python-requires = >=3.6
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology