Make the return type of multiprocessing.connection.Pipe more precise (#8706)

The precise return type depends on the platform. Link to implementation:
68fb03249f/Lib/multiprocessing/connection.py (L518)

Now users no longer need to use the internal-looking class
`_ConnectionBase` in annotations, at least in code that doesn't
need to be cross-platform.

stdlib/multiprocessing/connection.pyi

@@ -58,4 +58,12 @@ def wait(
     object_list: Iterable[Connection | socket.socket | int], timeout: float | None = ...
 ) -> list[Connection | socket.socket | int]: ...
 def Client(address: _Address, family: str | None = ..., authkey: bytes | None = ...) -> Connection: ...
-def Pipe(duplex: bool = ...) -> tuple[_ConnectionBase, _ConnectionBase]: ...
+
+# N.B. Keep this in sync with multiprocessing.context.BaseContext.Pipe.
+# _ConnectionBase is the common base class of Connection and PipeConnection
+# and can be used in cross-platform code.
+if sys.platform != "win32":
+    def Pipe(duplex: bool = ...) -> tuple[Connection, Connection]: ...
+
+else:
+    def Pipe(duplex: bool = ...) -> tuple[PipeConnection, PipeConnection]: ...

stdlib/multiprocessing/context.pyi

@@ -4,7 +4,6 @@ from collections.abc import Callable, Iterable, Sequence
 from ctypes import _CData
 from logging import Logger
 from multiprocessing import popen_fork, popen_forkserver, popen_spawn_posix, popen_spawn_win32, queues, synchronize
-from multiprocessing.connection import _ConnectionBase
 from multiprocessing.managers import SyncManager
 from multiprocessing.pool import Pool as _Pool
 from multiprocessing.process import BaseProcess
@@ -12,6 +11,11 @@ from multiprocessing.sharedctypes import SynchronizedArray, SynchronizedBase
 from typing import Any, ClassVar, TypeVar, overload
 from typing_extensions import Literal, TypeAlias
 
+if sys.platform != "win32":
+    from multiprocessing.connection import Connection
+else:
+    from multiprocessing.connection import PipeConnection
+
 if sys.version_info >= (3, 8):
     __all__ = ()
 else:
@@ -43,7 +47,15 @@ class BaseContext:
     def active_children() -> list[BaseProcess]: ...
     def cpu_count(self) -> int: ...
     def Manager(self) -> SyncManager: ...
-    def Pipe(self, duplex: bool = ...) -> tuple[_ConnectionBase, _ConnectionBase]: ...
+
+    # N.B. Keep this in sync with multiprocessing.connection.Pipe.
+    # _ConnectionBase is the common base class of Connection and PipeConnection
+    # and can be used in cross-platform code.
+    if sys.platform != "win32":
+        def Pipe(self, duplex: bool = ...) -> tuple[Connection, Connection]: ...
+    else:
+        def Pipe(self, duplex: bool = ...) -> tuple[PipeConnection, PipeConnection]: ...
+
     def Barrier(
         self, parties: int, action: Callable[..., object] | None = ..., timeout: float | None = ...
     ) -> synchronize.Barrier: ...