Add mypy to lint

An extremely modest beginning, but we'll roll this out over the whole codebase
in time.

Also move flake8 and mypy dependencies into setup[dev].

setup.py

@@ -91,6 +91,9 @@ setup(
         ':sys_platform != "win32"': [
         ],
         'dev': [
+            "flake8>=2.6.2, <3.1",
+            "mypy-lang>=0.4.5, <4.6",
+            "rstcheck>=2.2, <3.0",
             "tox>=2.3, <3",
             "mock>=2.0, <2.1",
             "pytest>=3, <3.1",

tox.ini

@@ -18,9 +18,7 @@ changedir = docs
 commands = sphinx-build -W -b html -d {envtmpdir}/doctrees . {envtmpdir}/html
 
 [testenv:lint]
-deps =
-  flake8>=2.6.2, <3.1
-  rstcheck>=2.2, <3.0
 commands =
   flake8 --jobs 8 --count mitmproxy pathod examples test
-  rstcheck README.rst
+  rstcheck README.rst
+  mypy -s ./mitmproxy/addonmanager.py