CI run against latest deps is failing

[github-actions]

See https://github.com/matrix-org/synapse/actions/runs/6026417807

[DMRobertson]

Oh, this has been failing for three weeks.

Run poetry run mypy
synapse/handlers/initial_sync.py:203: error: Argument 1 to "addCallback" of "Deferred" has incompatible type "Callable[[Dict[str, Mapping[Tuple[str, str], EventBase]]], Mapping[Tuple[str, str], EventBase]]"; expected "Callable[[Dict[str, Mapping[Tuple[str, str], EventBase]]], Failure]"  [arg-type]
synapse/handlers/initial_sync.py:203: error: Redundant cast to "Mapping[Tuple[str, str], EventBase]"  [redundant-cast]
synapse/handlers/initial_sync.py:203: error: Incompatible return value type (got "Mapping[Tuple[str, str], EventBase]", expected "Failure")  [return-value]
Found 3 errors in 1 file (checked 853 source files)

We might need to suppress redundant casts like we do in the twisted trunk job---but there's more going on here.

[DMRobertson]

...that was the wrong issue

[DMRobertson]

The redundant cast is

synapse/synapse/handlers/initial_sync.py

Line 203 in 41f127e

lambda states: cast(StateMap[EventBase], states[event.event_id])

which mypy can deduce for itself using the latest twisted annotations. If we make mypy ignore the redundant cast, or remove the cast entirely, then it passes. So I think the extra complains that mypy makes are false positives---something to do with mypy's overload resolution?

Fix is to port over the ignore-redundant-casts change from #16121

Later, when we bump the lockfile to use the latest twisted, we can remove the cast entirely.