Add in-tree upper-constraints for py35 job

It is difficult to keep the py35 upper constraints on reqiorements
repo's upper-constarint.txt file as update-constraint tools generate
the new file everytime with official supported python version only
which is py3.6 and py3.8 for Victoria cycle
- https://review.opendev.org/#/c/730428

Let's keep the py3.5 constraints in-tree which can be used in py35
unit test job.

Change-Id: I397edb7eccdb23656d1a618635d9cb9c0c901d39

py35-upper-constraints.txt

@@ -0,0 +1 @@
+zipp===1.1.0

tox.ini

@@ -14,6 +14,12 @@ deps =
 commands =
   stestr run --slowest {posargs}
 
+[testenv:py35]
+deps =
+  -c{toxinidir}/py35-upper-constraints.txt
+  -r{toxinidir}/requirements.txt
+  -r{toxinidir}/test-requirements.txt
+
 [testenv:integration]
 whitelist_externals = bash
 commands =