update the versions of python 3 claimed

We're running at least 3.5 in CI, so start there.

Update tox to set the environments from newest to oldest to avoid
issues with the dbm format selected for the .testrepository database
generated by the tests, to allow it to work for all tox environments
on a local system.

Change-Id: I7cde15039f782bba69ed9e0e44b08e289148019e
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

setup.cfg

@@ -12,8 +12,9 @@ classifier =
     License :: OSI Approved :: Apache Software License
     Operating System :: POSIX :: Linux
     Programming Language :: Python
-    Programming Language :: Python :: 3.3
-    Programming Language :: Python :: 3.4
+    Programming Language :: Python :: 3.5
+    Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
 
 [global]
 setup-hooks =

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 1.6
-envlist = py33,py34,pep8
+envlist = py37,py36,py35,pep8
 
 [testenv]
 install_command = pip install -U {opts} {packages}