Move from py35 to py37 for gating py3 jobs

Looking at https://governance.openstack.org/tc/reference/runtimes/train.html
OpenStack is no longer targetting Python 3.5, so it doesn't really make sense
to gate on it. Move to Python 3.7, just to make it so we can go longer before
having to do this shuffle again.

Presumably, once we actually declare support for Python 3.6, we should
make that job voting, too? We've had enough gate flaky-ness lately that
I'm not keen on it.

Change-Id: I9c9e5df76a6592ab9c512b7c1ba807f9a0053059

.zuul.yaml

@@ -327,6 +327,7 @@
             irrelevant-files:
               - ^(api-ref|doc|releasenotes)/.*$
               - ^test/(functional|probe)/.*$
+            voting: false
         - swift-tox-py36:
             irrelevant-files:
               - ^(api-ref|doc|releasenotes)/.*$
@@ -336,7 +337,6 @@
             irrelevant-files:
               - ^(api-ref|doc|releasenotes)/.*$
               - ^test/(functional|probe)/.*$
-            voting: false
         - swift-tox-func:
             irrelevant-files:
               - ^(api-ref|doc|releasenotes)/.*$
@@ -408,7 +408,7 @@
         # long-running jobs, like probetests or (once they move to
         # in-tree definitions) dsvm jobs.
         - swift-tox-py27
-        - swift-tox-py35
+        - swift-tox-py37
         - swift-tox-func
         - swift-tox-func-encryption
         - swift-tox-func-domain-remap-staticweb

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py35,py27,pep8
+envlist = py37,py27,pep8
 minversion = 2.3.2
 skipsdist = True
 
@@ -32,7 +32,7 @@ setenv = VIRTUAL_ENV={envdir}
 # tests file by file, add them into the list below, and never go back.
 # This list also serves as a shared task board for those helping with py3.
 # But mind that reviews expanding this list may be outstanding in Gerrit.
-[testenv:py35]
+[testenv:py37]
 commands =
   nosetests {posargs:\
       test/unit/account \
@@ -96,10 +96,10 @@ commands =
       test/unit/proxy/controllers/test_obj.py}
 
 [testenv:py36]
-commands = {[testenv:py35]commands}
+commands = {[testenv:py37]commands}
 
-[testenv:py37]
-commands = {[testenv:py35]commands}
+[testenv:py35]
+commands = {[testenv:py37]commands}
 
 [testenv:pep8]
 basepython = python2.7