mathlib

ce999a89 - feat(topology/basic): add `is_open_iff_ultrafilter` (#4529)

feat(topology/basic): add `is_open_iff_ultrafilter` (#4529) Requested on [Zulip](https://leanprover.zulipchat.com/#narrow/stream/217875-Is-there.20code.20for.20X.3F) by Adam Topaz