Add calling() method to CircuitBreaker, returning a context manager

Summary

This adds a new calling() method to CircuitBreaker. This method returns a context manager, enabling the circuit breaker to be used with a with statement. The block of code inside the with statement will be executed according to the rules implemented by the current state of the circuit breaker.

Fixes: https://github.com/danielfm/pybreaker/issues/85

Example usage:

    db_breaker = CircuitBreaker()

    # Will trigger the circuit breaker
    with db_breaker.calling():
        # Do stuff here...
        pass

Implementation Details

Initially, I attempted to make the CircuitBreaker itself a context manager, by implementing __enter__ and __exit__ in the class itself, but this required an amount of refactoring that I was not comfortable with. Therefore, I opted to implement this on top of the existing call method.

As for the naming of the new method, I took inspiration from PEP 0343, where the authors suggested using paste tense ("-ed") or progressive tense ("-ing"). Even though the new method itself isn't calling the with block itself, conceptually, it can be thought of doing so.

Test Plan

Added a test case that exercises the new API.

danielfm / pybreaker