This is certainly not something that requires language support. It can easily be purely a convention, as long as different IDEs, linters, type checkers, etc. agree on what the convention is. Maybe at some point in the future, if the convention becomes adopted, there might be some help in having a standard library module, or even minimal language recognition, of the convention. But let's work on adopting a convention first.
Yes, this sounds good. There is no need to change the python language, it is should be a convetion.
Might not even be a convention. I have now tried using https://github.com/edreamleo/make-stub-files https://github.com/edreamleo/make-stub-files on the code base at work. I had to fix some fatal bugs, and it's still pretty buggy even after those fixes, AND PyCharm has some nasty bugs with stub files that are blockers, but it's clear that generating stub files to do this is feasible.