tox: Add 'mypy' target

Like the recently added 'docs' target, this is easier than creating a
virtualenv, installing deps and running make.

Signed-off-by: Stephen Finucane <stephen@that.guru>

tox.ini

@@ -55,6 +55,12 @@ commands=
     {envpython} -Wall tests/run.py {posargs}
     sphinx-build -q -W -b html -d {envtmpdir}/doctrees doc {envtmpdir}/html
 
+[testenv:mypy]
+deps=
+    mypy
+commands=
+    mypy sphinx/
+
 [testenv:docs]
 commands=
     python setup.py build_sphinx