_posixsubprocess is not available on Windows (#6737)

stdlib/_posixsubprocess.pyi

@@ -1,24 +1,26 @@
 # NOTE: These are incomplete!
 
+import sys
 from typing import Callable, Sequence
 
-def cloexec_pipe() -> tuple[int, int]: ...
-def fork_exec(
-    args: Sequence[str],
-    executable_list: Sequence[bytes],
-    close_fds: bool,
-    fds_to_keep: Sequence[int],
-    cwd: str,
-    env_list: Sequence[bytes],
-    p2cread: int,
-    p2cwrite: int,
-    c2pred: int,
-    c2pwrite: int,
-    errread: int,
-    errwrite: int,
-    errpipe_read: int,
-    errpipe_write: int,
-    restore_signals: int,
-    start_new_session: int,
-    preexec_fn: Callable[[], None],
-) -> int: ...
+if sys.platform != "win32":
+    def cloexec_pipe() -> tuple[int, int]: ...
+    def fork_exec(
+        args: Sequence[str],
+        executable_list: Sequence[bytes],
+        close_fds: bool,
+        fds_to_keep: Sequence[int],
+        cwd: str,
+        env_list: Sequence[bytes],
+        p2cread: int,
+        p2cwrite: int,
+        c2pred: int,
+        c2pwrite: int,
+        errread: int,
+        errwrite: int,
+        errpipe_read: int,
+        errpipe_write: int,
+        restore_signals: int,
+        start_new_session: int,
+        preexec_fn: Callable[[], None],
+    ) -> int: ...