lint: Fix mypy warnings

Author: dalcinl

conf/mypy.stubtest.allow.txt

@@ -1,2 +1 @@
-mpi4py.MPI
 mpi4py.futures._base.*

src/mpi4py/util/pkl5.pyi

@@ -1,6 +1,6 @@
 from __future__ import annotations
 from .. import MPI
-from ..MPI import ANY_SOURCE, ANY_TAG
+from ..MPI import PROC_NULL, ANY_SOURCE, ANY_TAG
 from ..MPI import Status, Datatype
 from typing import Any, Literal, Optional
 from typing import Callable, Iterable, Sequence