Essential
python-dev and python-committers are where most activity happens. You need to be there to be effective.
Discourse is where a group of us wanted that activity to move but this fell short: the improvements of Discourse weren’t enough to convince mailing list aficionados to switch, since there is some inevitable adaptation curve and perceived feature loss (see xkcd://1172).
Some core developers do prefer Discourse though and post on it more often than on the mailing lists. One particularly popular feature is polling core devs for stuff. So it pays to be here, too.
It’s not great to have two channels and this was discussed this week. No decisions AFAICT.
Helpful
Python Core Devs Discord is pretty confusingly similarly named to Discourse so people keep confusing the two in conversations. What it is is a realtime chat to talk to other core devs. It’s invite-only, available to core devs, triagers, and mentees only. Since it’s not open and timezones make it impossible for everybody to interact on equal terms, nothing really gets decided on it. But if you want to pick somebody’s brain quickly, or poke them to review your pull request, or rant about Linux distributions not using PIE, or show off your new widescreen monitor, this is the place to be 
Note that the Python Core Devs Discord is separate from the massively popular user-oriented Python Discord. I’m there too but it’s unrelated to core development work.
Not important
IRC is legacy. There is few people there and unless they got some Matrix integration, conversations get interrupted and lost when people disconnect. We keep it, I’m on it, but I think it’s fair to say that it died with Freenode.
Zulip is kind of a sad story since it’s written in Python, open source, and kind of fills the same space as Discord. Sadly it didn’t win people’s hearts and so multiple times we discussed whether to continue using it and we finally decided that Zulip failed.