feat: Add initial type hints for RedisSession

[riton]

First of all, thanks for providing a maintained alternative to the flask-session project 👍

This P.R is a proposal for type hinting this project.

I've currently only added type hints to the Redis session implementation.

I wanted to initiate a discussion with you before going further on that path.

Regards

[riton]

I can see that the original project has been adopted by Pallets Community and that you're trying to merge back your modifications into original project. Nice move 👍 👏

I should reopen this P.R against original project, don't you think ?

[christopherpickering]

hey @riton thanks for this, sorry I missed it!

You could pr the original project as well. I don't use this anymore and probably won't be helping much there... it seems like some people just wanted that project updated for the latest flask but don't care about other fixes/improvements.

This looks good, you can undraft it and I will merge it in.