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

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

constraints.txt

@@ -0,0 +1 @@
+pywin32!=226

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 minversion = 3.4.0
 skipsdist = True
-envlist = py{36,37}, docs, docs-links, flake8-nb
+envlist = py{36,37,38}, docs, docs-links, flake8-nb
 
 [flake8]
 max-line-length = 99
@@ -10,23 +10,26 @@ max-line-length = 99
 [testenv:docs]
 whitelist_externals = make
 commands =
-    make --directory=docs clean html
+  make --directory=docs clean html
 
 [testenv:docs-links]
 whitelist_externals = make
 commands =
-    make --directory=docs clean linkcheck
+  make --directory=docs clean linkcheck
 
 
 [testenv:flake8-nb]
 basepython=python
-deps=flake8-nb
+skip_install=true
+commands_pre =
+  {envpython} -m pip install -U -q -c {toxinidir}/constraints.txt flake8-nb
 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 -q -r {toxinidir}/requirements_dev.txt
+  {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
+  pytest --nbval --current-env