Add support for passing full objects instead of IDs to custom methods

[ob-stripe]

r? @brandur-stripe
cc @stripe/api-libraries

See my comment here for context around the issue this fixes.

In Python, instance methods are really functions that accept the instance as their first argument, e.g.:

class Foo(object):
    def do(self):
        print("Hi!")

foo = Foo()

# the following are equivalent
foo.do()
Foo.do(foo)

The hack we introduced in #543 to support declaring instance methods and class methods with the same name broke support for the second syntax. This PR restores support for the second syntax by explicitly checking if the method was invoked with an instance as its first argument.

Fixes #597.

[brandur-stripe]

Great work on this PR — thank you for the comments (made things very clear), and thank you for the tests (let's me assume that this actually works ;).

LGTM.

[ob-stripe]

Released as 2.33.2.