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.

stdlib/2and3/contextlib.pyi

@@ -1,12 +1,10 @@
-# Stubs for contextlib
-
+# ContextManager aliased here for backwards compatibility; TODO eventually remove this
 from typing import (
     Any, Callable, Generator, IO, Iterable, Iterator, Optional, Type,
     Generic, TypeVar, overload
 )
 from types import TracebackType
 import sys
-# Aliased here for backwards compatibility; TODO eventually remove this
 from typing import ContextManager as ContextManager
 
 if sys.version_info >= (3, 5):

stdlib/2and3/shutil.pyi

@@ -1,10 +1,6 @@
 import os
 import sys
 
-# 'bytes' paths are not properly supported: they don't work with all functions,
-# sometimes they only work partially (broken exception messages), and the test
-# cases don't use them.
-
 from typing import (
     List, Iterable, Callable, Any, Tuple, Sequence, NamedTuple,
     AnyStr, Optional, Union, Set, TypeVar, overload, Type, Protocol, Text

stdlib/2and3/sysconfig.pyi

@@ -1,12 +1,10 @@
-# Stubs for sysconfig
-
 from typing import overload, Any, Dict, IO, List, Optional, Tuple, Union
 
+def get_config_var(name: str) -> Optional[str]: ...
 @overload
 def get_config_vars() -> Dict[str, Any]: ...
 @overload
 def get_config_vars(arg: str, *args: str) -> List[Any]: ...
-def get_config_var(name: str) -> Optional[str]: ...
 def get_scheme_names() -> Tuple[str, ...]: ...
 def get_path_names() -> Tuple[str, ...]: ...
 def get_path(name: str, scheme: str = ..., vars: Optional[Dict[str, Any]] = ..., expand: bool = ...) -> Optional[str]: ...