Support for PTB v20.0a0

[Bibo-Joshi]

PTB v20.0a0 was just released. Many of the contributions in ptbcontrib will not work with that version. For now, the requirements where all updated to reflect that (see #42), but in the long run it would be nice to have the contributions updated to v20.0a0.

I'm pinging all original contributors here to inform them. If you have no interest in updating your contribution and/or want to wait with that until v20 gets a stable release, that's of course no problem!

@allerter @starry69 @Poolitzer @bqback