Add @staticmethod overloads

[richardm-stripe]

Fixes #1113.

As #1113 reports, the types for our classmethods do not work for MyPy. The problem is our @class_method_variant pattern and the fact that we have methods with the same name that double as both class methods and instance methods.

MyPy doesn't like it when you do not apply @classmethod consistently to all overloads of a method. It complains

Overload does not consistently use the "@classmethod" decorator on all function signatures.  [misc]

I don't mind doing something MyPy doesn't like internally in stripe-python, so long as it gives users types that accurately describe correct usage of the library, but unfortunately in this case MyPy will not generate the overload type that omits self or cls (unlike how Pyright behaves, incidentally). Here's a simplified example:

class Foo:
    @overload
    def boop(self, id: str) -> None:
        pass

    @overload
    @classmethod
    def boop(cls, id: str) -> None:
        pass

    def boop(*args: Any, **kwargs: Any) -> None:
        pass

reveal_type(Foo.boop)
# MYPY: Revealed type is "Overload(def (self: t.Foo, id: builtins.str), def (cls: Type[t.Foo], id: builtins.str))"
# PYRIGHT: Type of "Foo.boop" is "Overload[(self: Foo, id: str) -> None, (id: str) -> None]"

reveal_type(Foo().boop)
# MYPY: Revealed type is "def (id: builtins.str)"
# PYRIGHT: Type of "Foo().boop" is "Overload[(id: str) -> None, (id: str) -> None]"

This PR addresses the problem by adding an additional @staticmethod overload. This isn't very satisfying, as @staticmethod is for methods that do not actually need a reference to cls. On the other hand, because we're only using it on an overload, it doesn't really change the way anything behaves at runtime at all, and it causes MyPy to generate an overload typed the way we want, i.e.:

reveal_type(Foo.boop)
# Revealed type is "Overload(def (self: t.Foo, id: builtins.str), def (cls: Type[t.Foo], id: builtins.str), def (id: builtins.str))"
reveal_type(Foo().boop)
# Revealed type is "def (id: builtins.str)"

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