BUG: hotfix for a functional breakage introduced in #3546

[neutrinoceros]

PR Summary

This should resolve the docs part of #3690

For reproduction:

import yt

ds = yt.load_sample("IsolatedGalaxy")
p = yt.SlicePlot(ds, "z", "density")
p.annotate_title("test")
p.save("/tmp/")

[matthewturk]

OK, so why is this now a bug, when it wasn't before, and when we explicitly said all the typing stuff would be optional? I'm confused, and this needs to be figured out before we do anything else.

[neutrinoceros]

This is because #3546 never triggered the docs build, and apparently the only tests we have that exercise what's broken here are in there

[matthewturk]

@neutrinoceros Why is this an error if it's just typing? Because of mypy?

[matthewturk]

So the Optional is so that it's not required to be defined in subclasses, right? Not that it isn't required to be defined as an attribute? This is rather frustrating if this is the case, because if that is correct, it seems like it may be pointless to have anything in a base class that is typed as anything other than Optional. Is that the case?

[neutrinoceros]

It's not. Optional[Tuple] is short for Union[None, Tuple], but the name is indeed confusing. It's going away in Python 3.10 where we can replace it with None | Tuple, so we don't have to live with it for ever.

[neutrinoceros]

It's not just typing, apparently I removed the default value for PlotCallback._supported_geometries in my typing PR. I was expecting CI to call me out when I broke things (and indeed, it happened a couple times), but it seems like this slipped in still

[matthewturk]

I see, ok.