tox: Enable testing python 3.6.0

The requirements_test.txt includes packages such as black which are
not compatible with Python 3.6.0. Switch to requirements_test_min.txt
so that `tox -e py36` works with 3.6.0.

Author: pkolbus

tox.ini

@@ -29,7 +29,7 @@ commands =
 setenv =
     COVERAGE_FILE = {toxinidir}/.coverage.{envname}
 deps =
-    -r {toxinidir}/requirements_test.txt
+    -r {toxinidir}/requirements_test_min.txt
 commands =
     ; Run tests, ensuring all benchmark tests do not run
     pytest --benchmark-disable {toxinidir}/tests/ {posargs:}