add tests with external mock library

tox.ini

@@ -137,6 +137,11 @@ commands =
 setenv =
     {[testenv:oldest]setenv}
 
+[testenv:external-mock]
+commands =
+    python {toxinidir}/tools/pip_install.py mock
+    {[base]install_and_test} {[base]all_packages}
+
 [testenv:lint]
 basepython = python3
 # separating into multiple invocations disables cross package