Add helper method for backoff

[cjrh]

For example, a backoff sequence could be produced like this:

        backoffs = itertools.chain(
            [0.1, 0.5, 1, 2, 5, 10], itertools.repeat(30, 100)
        )

This is a handy thing to have lying around. We could even consider making a retry handler, although that is probably out of scope for this package and should go into a different package.