Replace `float` with `float | int`

As discussed in python/typing#1748: I propose to (slowly) replace float with float | int and complex with complex | float | int, where applicable.

• It's more correct, even if float is current implicitly equivalent to int | float.
• We are already prepared should the implicit promotion ever be removed.
• We give type checkers the ability to optionally disable the promotion so that type checker authors and users can experiment with it.
• Typeshed would always need to be the first step for this, so let's remove this blocker.

This issue proposes an ugly and slow, but correct and safe procedure:

1. Disable flake8-pyi code Y041.
   #16096
2. Introduce two type aliases FloatInt = float | int and ComplexInt = complex | float | int to _typeshed.
   #16068
3. Globally replace all instances of float with FloatInt and of complex with ComplexInt in the stdlib.
4. After 2026-09-22 we do the same for third-party stubs.
5. Review FloatInt and ComplexInt instances are replace them with the proper types. (This will take a while.)
6. Remove the type aliases once they are not needed anymore (in 10 years or so ...)

Using FloatInt and ComplexInt would be disallowed for new code and the type aliases should explicitly be marked as not to be used by non-typeshed code.

We should also probably add flake8-pyi checks that checks that argument types use float | int and complex | float | int. Cases where only float/complex is allowed are probably rare.

Start with the stdlib stubs and the _typeshed FloatInt and ComplexInt aliases, then inspect the flake8-pyi Y041 configuration. Search for float and complex annotations across the stdlib and review each candidate; completion means the planned stdlib-wide migration, while third-party stubs remain deferred until 2026-09-22.