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

This allows a lot more modes.

Which should be ok, since the mode parameter is just passed on to open(). We could argue not to allow read-only modes, but those were allowed previously, and I wanted to avoid semantic changes for now.

This is blocking two other PRs (#10232 and #10229), so it would be great if someone could have a look at it.