tox: add env to check package long_description using 'twine check' command

Restrucutred Text is very picky. 3.32.0 release process failed because of some broken rst.
I have to stop doing [skip ci] when I update the changelog... :(

tox.ini

@@ -37,6 +37,15 @@ commands =
     coverage combine
     codecov --env TOXENV
 
+[testenv:package_readme]
+description = check that the long description is valid (need for PyPi)
+deps = twine >= 1.12.1
+       pip >= 18.0.0
+skip_install = true
+extras =
+commands = pip wheel -w {envtmpdir}/build --no-deps .
+           twine check {envtmpdir}/build/*
+
 [testenv:bdist]
 deps =
     pygments