Add a tox env for building docs with python3

Author: g1itch

tox.ini

@@ -12,7 +12,7 @@ commands =
     coverage run -a src/bitmessagemain.py -t
     coverage run -a -m tests
 
-[testenv:py27-doc]
+[testenv:doc]
 deps =
     .[docs]
     -r docs/requirements.txt