tox: Make everything work with Python 3

Nothing too fancy here except that we've to work around a really ugly
side-effect of blockdiag.

Change-Id: Ibd32d30aacae65702d0ccbdb8a02b1667ec4e8ee

doc/source/conf.py

@@ -181,13 +181,26 @@ def monkey_patch_blockdiag():
     text width rather than on word boundaries. There's a patch submitted to
     resolve this [1]_ but it's unlikely to merge anytime soon.
 
+    In addition, blockdiag monkey patches a core library function,
+    ``codecs.getreader`` [2]_, to work around some Python 3 issues. Because
+    this operates in the same environment as other code that uses this library,
+    it ends up causing issues elsewhere. We undo these destructive changes
+    pending a fix.
+
     TODO: Remove this once blockdiag is bumped to 1.6, which will hopefully
     include the fix.
 
     .. [1] https://bitbucket.org/blockdiag/blockdiag/pull-requests/16/
+    .. [2] https://bitbucket.org/blockdiag/blockdiag/src/1.5.3/src/blockdiag/utils/compat.py # noqa
     """
+    import codecs
+    from codecs import getreader
+
     from blockdiag.imagedraw import textfolder
 
+    # oh, blockdiag. Let's undo the mess you made.
+    codecs.getreader = getreader
+
     def splitlabel(text):
         """Split text to lines as generator.
 

tox.ini

@@ -138,9 +138,6 @@ deps =
 commands = {posargs}
 
 [testenv:docs]
-# TODO(melwitt): This can be removed when the docs target can be run
-# with python 3.x
-basepython = python2.7
 deps = -r{toxinidir}/doc/requirements.txt
 commands =
   rm -rf doc/source/api doc/build api-guide/build api-ref/build placement-api-ref/build