CallableType.copy_modified: accept sequence for arg_names

[ikonst]

In #4070, CallableType.__init__ was changed to accept arg_names: Sequence[str | None] so we could pass e.g. list[str] to it. We're making a similar change to CallableType.copy_modified.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[JukkaL]

This may later have some performance implications, since Sequence operations are currently slower than list operations in compiled code. However, since this is using "bogus" types, there is no impact right now (bogus types are treated as Any by mypyc), and this change fine. I'm leaving this as a note for future readers that might look at this PR.

[ikonst]

@JukkaL Curious, does this matter given that the sequence is copied by list() in init?

[JukkaL]

A good point, it doesn't make a difference unless we change also __init__ to accept only concrete types.