[Tests] Specify module versions to fix tox tests

tox.ini

@@ -98,6 +98,8 @@ deps =
     {[testenv]deps}
     flake8
     flake8-quotes
+# Pin isort version due to bug in 4.2.13, failing checks only on travis!
+    isort==4.2.5
     flake8-isort
     pep8-naming
 commands =
@@ -175,6 +177,8 @@ deps =
     {[testenv]deps}
     sphinx
     sphinxcontrib-napoleon
+# Pin requests version due to bug: https://github.com/requests/requests/issues/4125
+    requests==2.15.1
 whitelist_externals =
     {[testenv]whitelist_externals}