Use 'python3' instead of 'python3.6' on tox

This allows us to use python3.7+ to use tox

tox.ini

@@ -77,7 +77,7 @@ commands =
 [testenv:regen]
 changedir = doc/en
 skipsdist = True
-basepython = python3.6
+basepython = python3
 deps =
     sphinx
     PyYAML
@@ -103,7 +103,7 @@ commands =
 
 [testenv:release]
 decription = do a release, required posarg of the version number
-basepython = python3.6
+basepython = python3
 usedevelop = True
 passenv = *
 deps =
@@ -116,7 +116,7 @@ commands = python scripts/release.py {posargs}
 
 [testenv:publish_gh_release_notes]
 description = create GitHub release after deployment
-basepython = python3.6
+basepython = python3
 usedevelop = True
 passenv = GH_RELEASE_NOTES_TOKEN TRAVIS_TAG TRAVIS_REPO_SLUG
 deps =