[PATCH 4 of 5 V6] testing: allow Hypothesis to enable extensions

[timeless]

David R. MacIver wrote:
>  from hypothesis.errors import HypothesisException
> -from hypothesis.stateful import rule, RuleBasedStateMachine, Bundle
> +from hypothesis.stateful import (
> +    rule, RuleBasedStateMachine, Bundle, precondition)

Since you're not done iterating over this, can you make the initial
commit use the parens and only one import per line:

from hypothesis.stateful import (
    rule,
    RuleBasedStateMachine,
    Bundle,
)

And have this patch just add precondition?

(I may have mentioned this on irc, but here's a great example of why.)