setup.cfg: Various Python 3 fixes

Now that we've dropped Python 2 support, we shouldn't be stating that we
support universal (Python 2 and 3) wheels. We should also use the proper
setuptools machinery to prevent people accidentally installing
novaclient in a Python 2.7 environment. Resolve both issues, removing an
unused 'upload_sphinx' section in the process.

Change-Id: Icee145f44a42c233008b3328f52a3eec933101e0
Signed-off-by: Stephen Finucane <sfinucan@redhat.com>

setup.cfg

@@ -7,6 +7,7 @@ license = Apache License, Version 2.0
 author = OpenStack
 author-email = openstack-discuss@lists.openstack.org
 home-page = https://docs.openstack.org/python-novaclient/latest
+python-requires = >=3.6
 classifier =
     Development Status :: 5 - Production/Stable
     Environment :: Console
@@ -19,6 +20,8 @@ classifier =
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
+    Programming Language :: Python :: 3 :: Only
+    Programming Language :: Python :: Implementation :: CPython
 
 [files]
 packages =
@@ -28,9 +31,6 @@ packages =
 console_scripts =
     nova = novaclient.shell:main
 
-[upload_sphinx]
-upload-dir = doc/build/html
-
 [compile_catalog]
 domain = novaclient
 directory = novaclient/locale
@@ -44,6 +44,3 @@ input_file = novaclient/locale/novaclient.pot
 keywords = _ gettext ngettext l_ lazy_gettext
 mapping_file = babel.cfg
 output_file = novaclient/locale/novaclient.pot
-
-[wheel]
-universal = 1