CLEAN: Investigate whether any TypeVar that constraints to a list of types can change to being bound to a Union

[Dr-Irv]

See comment 3 in microsoft/pyright#5178 (comment)

The pd.Series class is making use of a constrained TypeVar. I'm not sure why this was done, but it's almost certainly not the right approach. A constrained TypeVar is rarely the right tool — and that's especially true when it comes to class-scoped type variables. There are many reasons why a constrained TypeVar is problematic, but to give you one example... A Series is able to contain heterogeneous types, but a constrained TypeVar doesn't permit something like Series[int | str]. I recommend that you switch from a constrained TypeVar to a regular TypeVar with a bound that is a union. Not only will this more correctly model the way Series works in pandas, it will also avoid the many bugs that mypy has with constrained TypeVars. I see that constrained TypeVars are used in a number of locations within the pandas stubs. I recommend looking at each of these uses and considering whether there's a better approach.

We should investigate whether we can change all TypeVar that have a list of types to refer to a Union of those types instead, with the bound argument. It works fine for S1 in #716