Allow sub-second backoff sleep

[ampledata]

https://github.com/ampledata/pytak/blob/4e8ad2377728b9ca34cf3d164b142d2fef4a20e5/pytak/classes.py#L59

Currently this code-block casts any sleep period to an int, which restricts us to either 0 sleep, or sleep >= 1 second. For more variability, as well as sub-second sleep times, this should be refactored to allow float and not force to int.