Fix return type of CancelScope.__exit__ such that it can suppresses errors

[rsokl]

The current return type of CancelScope.__exit__ is bool | None, which tells type checkers that CancelScope cannot suppress errors:

https://typing.readthedocs.io/en/latest/spec/exceptions.html#context-managers

If the return type of the exit method is specifically bool or Literal[True], a type checker should assume that exceptions can be suppressed. For any other return type, a type checker should assume that exceptions are not suppressed. Examples include: Any, Literal[False], None, and bool | None.

This is bad. If there were ever a context manager that could suppress errors, it would be CancelScope 😄

Given:

def f()
    with trio.move_on_after(10):
        while True:
            await trio.sleep(20)
            return 2
     return None

Before: pyright says the return type of f is int. I.e. it thinks the return None is unreachable
After: pyright says the return type of f is int | None, which is correct.

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[oremanj]

@rsokl Thank you for the fix, but please note that trio-typing is somewhat unloved now that trio comes with built-in support for type hints. Please consider using those instead - they were already correct in this respect :-)