Add __format__ overrides to complex, float, int, and str

[brianschubert]

These four builtins each define their own __format__ methods at runtime. While they all have the same signature as object.__format__, explicit overrides are needed so that type checkers resolve the right overrides in the presence of multiple inheritance.

Currently, type checkers incorrectly accept this code:

class Base:
    def __format__(self, format_spec: str) -> str: ...  # note: accepts keyword arguments

class Foo(int, Base): ...

# mypy:    ok
# pyright: ok
# PyCharm: ok
# runtime: TypeError: int.__format__() takes no keyword arguments
Foo().__format__(format_spec="02d")  

In particular, this affects enum.IntEnum/enum.StrEnum subclasses, which use int.__format__/str.__format__ at runtime, but type checkers think they use enum.Enum.__format__ (which, unlike the builtin __format__ methods, accepts keyword arguments).

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[brianschubert]

(for context, this was discovered as a common source of errors when running stubtest with python/mypy#19593)