Add mixins module to asyncio (#6789)

stdlib/VERSIONS

@@ -62,6 +62,7 @@ array: 2.7-
 ast: 2.7-
 asynchat: 2.7-
 asyncio: 3.4-
+asyncio.mixins: 3.10-
 asyncio.compat: 3.4-3.6
 asyncio.exceptions: 3.8-
 asyncio.format_helpers: 3.7-

stdlib/asyncio/mixins.pyi

@@ -0,0 +1,7 @@
+import threading
+from typing import NoReturn
+
+_global_lock: threading.Lock
+
+class _LoopBoundMixin:
+    def __init__(self, *, loop: NoReturn = ...) -> None: ...