Include missing typehints (mostly function return value typehints)

[jonded94]

Hey! Thank you very much for creating this library. I'd like to start using this library productively, but unfortunately, for quite a lot of functions, a return value type hint seems to be missing, especially when it's None.

Note that although pyright will assume None as the return type hint when the function is missing one, mypy will assume (more rightly, if I may add) Any, but also throw you an error if you're using mypy in "strict" mode (which one should, IMHO).

For example, this code

import portalocker

lock = portalocker.RedisLock("test123")
lock.acquire()
name = lock.client_name
lock.release()

will lead to mypy errors such as:

prtlck.py:6: error: Call to untyped function "release" in typed context  [no-untyped-call]

In this PR, I added function return value typehints to all functions I could find where this is missing.