complete multiprocessing.connection stub (#1716)

Adds additional public members based on https://github.com/python/cpython/blob/master/Lib/multiprocessing/connection.py#L10 and https://docs.python.org/3/library/multiprocessing.html#multiprocessing-listeners-clients.

I found some discrepancies in the docs while adding these stubs and filed python/cpython#4304 to address them.

stdlib/3/multiprocessing/connection.pyi

@@ -1,4 +1,29 @@
-from typing import Any, Optional
+from typing import Any, Iterable, List, Optional, Tuple, Type, Union
+import socket
+import sys
+import types
+
+# https://docs.python.org/3/library/multiprocessing.html#address-formats
+_Address = Union[str, Tuple[str, int]]
+
+def deliver_challenge(connection: Connection, authkey: bytes) -> None: ...
+def answer_challenge(connection: Connection, authkey: bytes) -> None: ...
+if sys.version_info >= (3, 3):
+    def wait(object_list: Iterable[Union[Connection, socket.socket, int]], timeout: Optional[float] = ...) -> List[Union[Connection, socket.socket, int]]: ...
+def Client(address: _Address, family: Optional[str] = ..., authkey: Optional[bytes] = ...) -> Connection: ...
+def Pipe(duplex: bool = ...) -> Tuple[Connection, Connection]: ...
+
+class Listener:
+    def __init__(self, address: Optional[_Address] = ..., family: Optional[str] = ..., backlog: int = ..., authkey: Optional[bytes] = ...) -> None: ...
+    def accept(self) -> Connection: ...
+    def close(self) -> None: ...
+    @property
+    def address(self) -> _Address: ...
+    @property
+    def last_accepted(self) -> Optional[_Address]: ...
+    if sys.version_info >= (3, 3):
+        def __enter__(self) -> Listener: ...
+        def __exit__(self, exc_type: Optional[Type[BaseException]], exc_value: Optional[BaseException], exc_tb: Optional[types.TracebackType]) -> None: ...
 
 class Connection:
     def close(self) -> None: ...