Use StrCollection in more places

[matrixbot]

This issue has been migrated from #14809.

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Using a Sequence[str] or Collection[str] is bad since a str also fits those and you usually want this to refer to a Set[str], List[str], Tuple[str, ...], etc.

In #14716 @reivilibre added a StrCollection type which we can re-use for this.

This has bit us a few times in the past but I'm failing to find references at the moment.

It would be good to audit Sequence[str] and Collection[str] and see if any of those should be updated.