More flexible Python version support for third-party packages

[donBarbos]

Some libraries (usually deprecated or simply poorly maintained like cysimdjson) don't have releases for the latest versions of Python. Maybe it's worth giving them the ability to have type stubs in typeshed. Currently the only way to have these types is to create a separate package.

This is similar to issue #10722, but I thought it was worth discussing separately since I haven't seen anyone mention an upper bound.

[JelleZijlstra]

Note that upper bounds on Requires-Python are controversial (https://discuss.python.org/t/requires-python-upper-limits/12663); uv apparently ignores them. So we probably shouldn't be using them.