Stop testing Python 2

It's Ussuri. We can *finally* stop testing Python 2 [1]. Time to party.
We don't attempt any cleanup but simply stop testing with Python 2,
indicate that we only support Python 3 via 'setup.cfg' and remove any
Python 2 only dependencies. Our 'tox.ini' is modified such that
'functional' now runs with 'python3', whatever that may point to, though
the gate will only use a versioned variant (currently
'functional-py36').

This should free up a significant amount of resources from the gate and
let us start using Python 3 idioms in our code. Win-win.

[1] https://governance.openstack.org/tc/resolutions/20180529-python2-deprecation-timeline.html#python2-deprecation-timeline

Change-Id: Ie1a0cbd82a617dbcc15729647218ac3e9cd0e5a9
Signed-off-by: Stephen Finucane <sfinucan@redhat.com>

setup.cfg

@@ -6,6 +6,7 @@ description-file =
 author = OpenStack
 author-email = openstack-discuss@lists.openstack.org
 home-page = https://docs.openstack.org/nova/latest/
+python-requires = >=3.6
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -13,11 +14,11 @@ classifier =
     License :: OSI Approved :: Apache Software License
     Operating System :: POSIX :: Linux
     Programming Language :: Python
-    Programming Language :: Python :: 2
-    Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
+    Programming Language :: Python :: 3 :: Only
+    Programming Language :: Python :: Implementation :: CPython
 
 [extras]
 osprofiler =