added py38 tests to tox envs and and prevented possible problem when

used on windows with 'winpy32==226' see mhammond/pywin32#1439

tox.ini

@@ -29,7 +29,7 @@ commands=flake8_nb material
 [testenv]
 install_command=python -m pip install -c {toxinidir}/constraints.txt {opts} {packages}
 commands_pre =
-  {envpython} -m pip install -U pip
+  {envpython} -m pip install -U  -c {toxinidir}/constraints.txt pip
   {envpython} -m pip install -U -q -c {toxinidir}/constraints.txt -r {toxinidir}/requirements_dev.txt
 commands =
   pytest --nbval --current-env