Enable flake8 tests

Ran both by Travis and locally. There's some setup required, README updated.
A few important Flake8 checks are still disabled, we're going to enable them as
soon as the stubs are fixed and we can reliably run Flake8 locally with Python
3.6.

requirements-tests-py3.txt

@@ -0,0 +1,4 @@
+mypy-lang>=0.4.6
+typed-ast>=0.6.1
+flake8>=3.2.1
+flake8-bugbear>=16.12.2