In Python 3:
- The `mmap` type is `Iterable[int]`, not `Iterable[bytes]`.
- The `read_byte` method returns an `int`, and the `write_byte` method only accepts an `int` as its first and only argument.
- The `__setitem__` method accepts any `ReadableBuffer` object, not just `bytes`.
In both Python 2 and 3:
- The `__delitem__` method always raises a `TypeError`, so the proper return type is `NoReturn`.
- The `mmap` type isn't generic, so I've simplified the stubs by removing the unnecessary `_mmap` class.
