Literal: always import from typing_extensions for simplicity (#4219)

stdlib/2and3/codecs.pyi

@@ -3,11 +3,7 @@ from typing import Any, BinaryIO, Callable, Generator, IO, Iterable, Iterator, L
 
 from abc import abstractmethod
 import types
-
-if sys.version_info < (3,) or sys.version_info >= (3, 8):
-    from typing import Literal
-else:
-    from typing_extensions import Literal
+from typing_extensions import Literal
 
 # TODO: this only satisfies the most common interface, where
 # bytes (py2 str) is the raw form and str (py2 unicode) is the cooked form.

stdlib/2and3/imaplib.pyi

@@ -1,15 +1,10 @@
-import imaplib
 import subprocess
 import sys
 import time
 from socket import socket as _socket
 from ssl import SSLSocket, SSLContext
 from typing import Any, Callable, Dict, IO, List, Optional, Pattern, Text, Tuple, Type, Union
-
-if sys.version_info >= (3, 8):
-    from typing import Literal
-else:
-    from typing_extensions import Literal
+from typing_extensions import Literal
 
 # TODO: Commands should use their actual return types, not this type alias.
 #       E.g. Tuple[Literal["OK"], List[bytes]]

stdlib/2and3/msilib/__init__.pyi

@@ -1,12 +1,7 @@
-
 import sys
 from typing import List, Tuple, Union, Set, Optional, Dict, Container, Any, Type, Iterable, Sequence
 from types import ModuleType
-
-if sys.version_info >= (3, 8):
-    from typing import Literal
-else:
-    from typing_extensions import Literal
+from typing_extensions import Literal
 
 if sys.platform == 'win32':
     from _msi import _Database

stdlib/2and3/socket.pyi

@@ -16,11 +16,7 @@ CPython C source: https://github.com/python/cpython/blob/master/Modules/socketmo
 #   adapted for Python 2.7 by Michal Pokorny
 import sys
 from typing import Any, BinaryIO, Iterable, List, Optional, Text, TextIO, Tuple, TypeVar, Union, overload
-
-if sys.version_info >= (3, 8):
-    from typing import Literal
-else:
-    from typing_extensions import Literal
+from typing_extensions import Literal
 
 
 # ----- Constants -----

stdlib/2and3/winsound.pyi

@@ -1,10 +1,6 @@
-
 import sys
 from typing import Optional, Union, overload
-if sys.version_info >= (3, 8):
-    from typing import Literal
-else:
-    from typing_extensions import Literal
+from typing_extensions import Literal
 
 if sys.platform == "win32":
     SND_FILENAME: int

stdlib/2and3/xml/etree/ElementTree.pyi

@@ -1,5 +1,3 @@
-# Stubs for xml.etree.ElementTree
-
 from typing import (
     Any,
     Callable,
@@ -21,13 +19,8 @@ from typing import (
     Union,
     overload,
 )
-import io
 import sys
-
-if sys.version_info < (3,) or sys.version_info >= (3, 8):
-    from typing import Literal
-else:
-    from typing_extensions import Literal
+from typing_extensions import Literal
 
 VERSION: str