Add mypy env

There are still kinks to be worked out but this works reasonably well.
This will fail until we can figure out how to import things from the
typing module for the type: comments only. We do not want to add a
dependency on that backport only for this mypy env.

flake8/style_guide.py

@@ -74,7 +74,7 @@ class StyleGuide(object):
         self._decision_cache = {}
 
     def is_user_selected(self, code):
-        # type: (Error) -> Union[Selected, Ignored]
+        # type: (str) -> Union[Selected, Ignored]
         """Determine if the code has been selected by the user.
 
         :param str code:
@@ -95,7 +95,7 @@ class StyleGuide(object):
         return Ignored.Implicitly
 
     def is_user_ignored(self, code):
-        # type: (Error) -> Union[Selected, Ignored]
+        # type: (str) -> Union[Selected, Ignored]
         """Determine if the code has been ignored by the user.
 
         :param str code:
@@ -123,7 +123,7 @@ class StyleGuide(object):
         return Decision.Ignored
 
     def should_report_error(self, code):
-        # type: (Error) -> Decision
+        # type: (str) -> Decision
         """Determine if the error code should be reported or ignored.
 
         This method only cares about the select and ignore rules as specified
@@ -159,6 +159,7 @@ class StyleGuide(object):
         return decision
 
     def is_inline_ignored(self, error):
+        # type: (Error) -> bool
         """Determine if an comment has been added to ignore this line."""
         # TODO(sigmavirus24): Determine how to handle stdin with linecache
         if self.options.disable_noqa:

tox.ini

@@ -35,6 +35,16 @@ deps =
 commands =
     pylint flake8
 
+[testenv:mypy]
+basepython = python3
+skipsdist = true
+skip_install = true
+use_develop = false
+deps =
+    mypy-lang
+commands =
+    mypy flake8
+
 [testenv:docs]
 deps =
     sphinx>=1.3.0