Added missing type annotations in various stdlib stubs. (#4402)

Co-authored-by: Eric Traut <erictr@microsoft.com>

stdlib/2and3/imaplib.pyi

@@ -3,6 +3,7 @@ import sys
 import time
 from socket import socket as _socket
 from ssl import SSLContext, SSLSocket
+from types import TracebackType
 from typing import IO, Any, Callable, Dict, List, Optional, Pattern, Text, Tuple, Type, Union
 from typing_extensions import Literal
 
@@ -60,7 +61,7 @@ class IMAP4:
     if sys.version_info >= (3, 5):
         def enable(self, capability: str) -> _CommandResults: ...
         def __enter__(self) -> IMAP4: ...
-        def __exit__(self, *args) -> None: ...
+        def __exit__(self, t: Optional[Type[BaseException]], v: Optional[BaseException], tb: Optional[TracebackType]) -> None: ...
     def expunge(self) -> _CommandResults: ...
     def fetch(self, message_set: str, message_parts: str) -> Tuple[str, _AnyResponseData]: ...
     def getacl(self, mailbox: str) -> _CommandResults: ...