add _weakref.pyi

builtins/2.7/_weakref.pyi

@@ -0,0 +1,16 @@
+from typing import Any, Callable
+
+class CallableProxyType(object):  # "weakcallableproxy"
+  pass
+
+class ProxyType(object):  # "weakproxy"
+  pass
+
+class ReferenceType(object):  # "weakref"
+  pass
+
+ref = ReferenceType
+
+def getweakrefcount(object: Any) -> int: ...
+def getweakrefs(object: Any) -> int: ...
+def proxy(object: Any, callback: Callable[[Any], Any] = ...) -> None: ...