Type hints for apnstruncate, apnspushkin, notifications, and gcmpushkin

[H-Shay]

Here is my first try at adding some type hints to the below files in Sygnal, and making very minor modifications to the code to silence some mypy errors (like assert is not None).
A very small handful of errors remain (less than 4 I think?) across the files I changed but silencing them seemed like it might require more modification to the code, and I wanted to make sure that everything I had here was correct before I started tackling those.
I think that these are mostly correct but I find typing to be a little difficult so it is entirely possible I have gotten some things wrong.

[reivilibre]

Thanks for tackling this!
I think you might be noticing that trying to bite a lot at once will pull you into everywhere else in the code because everything's linked together.
A couple of places where you've used cast I've tried to replace with actual checks — cast should be thought of as 'oi mypy, you're too stupid to see that I'm right so just trust me unconditionally' — in general, we try to avoid it.

[squahtx]

For the tuple index out of range complaints, we might want to define
Choppable = Union[Tuple[Literal["alert", "alert.body"]], Tuple[Literal["alert.loc-args"], int]]
(ie. Choppable is either ("alert",), ("alert.body",) or ("alert.loc-args", int))
then mypy should be clever enough to figure things out.

[reivilibre]

there are merge conflicts, otherwise very close to merge I think

[reivilibre]

lgtm otherwise

[reivilibre]

Thank you!