Don't modify return values of manager methods in place, modify a copy instead

[jkaikkosplk]

I have made things!

#2343 and #1917 seem to be caused by _process_dynamic_method modifying the return types of QuerySet methods, while they're still being referenced by the base QuerySet class's methods. This works fine the first time a method's type is resolved, but since it modifies the base QuerySet's return value too (if it's a Union or something like list[T]), it gets rid of the TypeVar on the base QuerySet class. When we try to resolve the return type again from another manager from another model, the Generic is no longer there and we end up with whatever first replaced that Generic.

Creating a copy seems to fix it, but then again, I have no idea what I'm doing 🤷

Related issues

• Closes Union return values with generics from queryset methods resolve wrong when the queryset is used on multiple models. #2343
• Closes Wrong QuerySet method typing from Iterable[T] argument #1917

[flaeppe]

Thank you!