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.

tests/flake8_test.py

@@ -0,0 +1,47 @@
+#!/usr/bin/env python3
+import pathlib
+import subprocess
+import sys
+
+PY2_ONLY_KEYWORDS = [
+    'StandardError',
+    'apply',
+    'basestring',
+    'buffer',
+    'cmp',
+    'coerce',
+    'execfile',
+    'file',
+    'intern',
+    'long',
+    'raw_input',
+    'reduce',
+    'reload',
+    'unichr',
+    'unicode',
+    'xrange',
+]
+
+root = pathlib.Path(__file__).parent.parent
+paths = list(sorted(str(p) for p in root.glob('**/*.pyi')))
+window = 0
+size = 100
+returncode = 0
+
+print('Running flake8 on {} .pyi files...'.format(len(paths)))
+
+while True:
+    chunk = paths[window:window + size]
+    if not chunk:
+        break
+
+    proc = subprocess.run(
+        ['flake8', '--builtins=' + ','.join(PY2_ONLY_KEYWORDS)] + chunk,
+    )
+    if proc.returncode:
+        print('flake8 run failed!')
+        sys.exit(1)
+
+    window += size
+
+print('flake8 run clean.')