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: Iec2eaa2b7e8df777b6988c2134e7771b9b1fb5b5

py35-upper-constraints.txt

@@ -0,0 +1,7 @@
+oslo.i18n===3.25.1
+debtcollector===1.22.0
+oslotest===3.9.0
+reno===2.11.3
+mox3===0.28.0
+oslo.config===7.0.0
+oslo.utils===3.42.1

tox.ini

@@ -18,6 +18,12 @@ whitelist_externals =
   bash
 basepython = python3
 
+[testenv:py35]
+deps =
+  -c{toxinidir}/py35-upper-constraints.txt
+  -r{toxinidir}/requirements.txt
+  -r{toxinidir}/test-requirements.txt
+
 [testenv:pep8]
 commands =
   flake8