Drop py35 jobs

Python 3.5 was the target runtime for the Rocky release. The current
target py3 runtime for Stein is Python 3.6, so there is no reason to
keep testing against the older version as mentioned here[1].

[1] https://governance.openstack.org/tc/reference/runtimes/stein.html

Change-Id: Ib585add30e8b34016a4af18d567a3613aaa74241

.zuul.yaml

@@ -100,7 +100,6 @@
       - nodejs4-jobs
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python35-jobs
       - openstack-python36-jobs
       - openstack-python37-jobs
       - periodic-stable-jobs

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = pep8,py27,py35,py3-dj111,releasenotes,npm
+envlist = pep8,py27,py3-dj111,releasenotes,npm
 minversion = 2.3.2
 skipsdist = True
 
@@ -22,11 +22,6 @@ commands =
     find . -type f -name "*.pyc" -delete
     bash {toxinidir}/tools/unit_tests.sh {envpython} {toxinidir} {posargs}
 
-[testenv:py35]
-setenv =
-  PYTHONUNBUFFERED = 1
-  {[testenv]setenv}
-
 [testenv:py3-dj21]
 basepython = python3
 commands =