add BaseListProxy.__imul__ (#13266)

Author: tungol

stdlib/@tests/stubtest_allowlists/common.txt

@@ -394,7 +394,6 @@ multiprocessing.dummy.RLock
 
 # These multiprocessing proxy methods have *args, **kwargs signatures at runtime,
 # But have more precise (accurate) signatures in the stub
-multiprocessing.managers.BaseListProxy.__imul__
 multiprocessing.managers.BaseListProxy.__len__
 multiprocessing.managers.BaseListProxy.__reversed__
 multiprocessing.managers.BaseListProxy.reverse

stdlib/multiprocessing/managers.pyi

@@ -129,6 +129,7 @@ class BaseListProxy(BaseProxy, MutableSequence[_T]):
     def __setitem__(self, s: slice, o: Iterable[_T], /) -> None: ...
     def __mul__(self, n: SupportsIndex, /) -> list[_T]: ...
     def __rmul__(self, n: SupportsIndex, /) -> list[_T]: ...
+    def __imul__(self, value: SupportsIndex, /) -> Self: ...
     def __reversed__(self) -> Iterator[_T]: ...
     def append(self, object: _T, /) -> None: ...
     def extend(self, iterable: Iterable[_T], /) -> None: ...