chore: restrict pubsub to < 2.0

[HemangChothani]

Fixes#272

For the passing current scenario restrict version in nox.py file and for the future added in synth.py file.

google-cloud-pubsub==2.0.0 version has dropped the support of python2, so once storage will drop the support following changes needs to be done:

• replace create_topic(self.topic_path) to create_topic(name=self.topic_path)

• replace get_iam_policy(self.topic_path) to get_iam_policy(request={"resource": self.topic_path})

• replace set_iam_policy(self.topic_path, policy) to set_iam_policy(request={"resource": self.topic_path, "policy": policy})

• replace delete_topic(self.topic_path) to delete_topic(request={"topic":self.topic_path})

[plamut]

This looks good, the dependency should indeed be pinned to exclude the next major version, as major versions are generally expected to contain breaking changes.