Use of generics in requests typing

Currently the requests typing makes heavy use of unions. For example the parameter type is:
https://github.com/python/typeshed/blob/f99e8b0615182bb0f17d9d126019d8d72c207ab3/stubs/requests/requests/sessions.pyi#L97-L103

The issue with this approach is that the type system can't keep track of which type has been chosen for a given field. For example, running pyright on this code:

from requests import Session

session = Session()
session.params = {}
reveal_type(session.params)

Shows us that the type system immediately "forgets" that our parameters are definitely in a dictionary:

test.py:5:13 - information: Type of "session.params" is "dict[str | bytes | int | float, str | bytes | int | float | Iterable[str | bytes | int | float] | None]"
0 errors, 0 warnings, 1 information

If we could make Session (as well as other classes such as Request) generic over the parameter type (as well as other fields), then we could keep track of this kind of state. For example, it might look like:

session: Session[dict] = Session()

Start with the linked requests/requests/sessions.pyi section around lines 97-103, then review how Session and Request are typed. Use the provided pyright example to assess whether generic parameter types remain specific after assignment; done means the proposed generic design is consistently defined for the affected classes and preserves the selected field types.