docstub for stm module - add MemoryObject class

[Josverl]

there are a few pyright errors in the stm docstub errors on getitem setitem not defined

#     Read/write 8 bits of memory.
mem8: Any = ...
#     Read/write 16 bits of memory.
mem16: Any = ...
#     Read/write 32 bits of memory.
mem32: int = 1

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class __MemoryObject():
    """
    A memory object.  This is a wrapper around a memory region.  It can be used
    to access the memory region as if it were an array of bytes.

    These memory objects can be used in combination with the peripheral register
    constants to read and write registers of the MCU hardware peripherals, as well
    as all other areas of address space.

    """
    ...

    def __getitem__(self, key: int) -> Any:
        ...
    def __setitem__(self, key: int, value: Any) -> None:
        ...

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