treat collection types in package as immutable

[dimbleby]

addresses #8250, #7459: but the interesting part of the fix is in poetry-core and I'll make more comments there

[dimbleby]

python-poetry/poetry-core#663 is the companion MR and, not for the first time, it will be awkward to find a sequence of merges that keeps both pipelines happy throughout

Edit: I take it back, it's possible to merge the poetry-core one first - because the pipelines in poetry-core don't run mypy over poetry proper

[radoering]

@dimbleby I suppose we have to update poetry-core in this PR to pass mypy. Do you have the time to take care of it?

[dimbleby]

gonna need to upgrade poetry core for lots of reasons though, right? I'd think that wants handling in a separate MR and then this one can simply merge / rebase

either way I wont' be doing anything before the weekend, probably can find time then though.

[radoering]

I'd think that wants handling in a separate MR and then this one can simply merge / rebase

I assumed that mypy would fail after upgrading poetry-core without the changes of this PR but I'll try if it's possible to do it separately or if other things break. (I have the hope that all other things are just deprecation warnings we can address later.)

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