Preparations for the Big Reformat (#4228)

A few comments between imports were removed or moved to the top of the
import block, due to behavioral differences between black and isort. See
psf/black#251 for details.

In two instances @overloads at the top of the file needed to be moved
due to psf/black#1490.

third_party/3/six/__init__.pyi

@@ -1,5 +1,3 @@
-# Stubs for six (Python 3.5)
-
 from __future__ import print_function
 
 from typing import (
@@ -26,7 +24,6 @@ import types
 import typing
 import unittest
 
-# Exports
 from io import StringIO as StringIO, BytesIO as BytesIO
 from builtins import next as next
 from functools import wraps as wraps