`SharedMemory.buf` should be optional (is `None` after `SharedMemory.close()`

[bersbersbers]

This code shows that SharedMemory.buf can be None:

from multiprocessing.shared_memory import SharedMemory

shm = SharedMemory(name="foo", create=True, size=1)
shm.close()
assert shm.buf is None

Yet the type hint says it cannot be None:

typeshed/stdlib/multiprocessing/shared_memory.pyi

Lines 17 to 18 in 703f7d0

	@property
	def buf(self) -> memoryview: ...

This leads mypy v1.18.1 to emit the new (optional)

Non-overlapping identity check (left operand type: "memoryview[int]", right operand type: "None")
See https://mypy.rtfd.io/en/stable/_refs.html#code-comparison-overlap for more infoMypy