update python requirements and pip usage

tox.ini

@@ -11,10 +11,12 @@
 #  and also to help confirm pull requests to this project.
 
 [tox]
-envlist = py{36,37,38}-{base,i18n},flake8,project,i18n,docs
+envlist = py{37,38,39}-{base,i18n},flake8,project,i18n,docs
 
 
 [testenv]
+setenv =
+    PIP_EXTRA_INDEX_URL=https://package-index.level12.net
 usedevelop = false
 deps =
     readme_renderer
@@ -40,7 +42,7 @@ commands =
 
 
 [testenv:project]
-basepython = python3.6
+basepython = python3.9
 skip_install = true
 usedevelop = false
 deps =
@@ -60,7 +62,7 @@ commands =
     make -C docs/ html
 
 [testenv:flake8]
-basepython = python3.7
+basepython = python3.9
 skip_install = true
 usedevelop = false
 deps = flake8