Fix: is_generator logic

[gabewillen]

Fix the is_generator logic to allow passing a callable object as a handler.

# simple example
import runpod


class Handler:

    def __call__(self, job):
        for x in range(10):
            yield {"message": f"Hello, World! {x}"}


runpod.serverless.start(
    {
        "handler": Handler(),
    }
)

[gabewillen]

Closing this as it's not absolutely necessary