Bite 25. No promo twice, keep state in a class

In this bite a real world scenario: PyBites has a growing set of Bites and gives away promos. They choose a Bite randomly but don't want to choose the same one again.

Hence you are provided with a BITES dict and a BITES_DONE set that we use (copies) in the Promo class via its constructor (code provided).

Complete the following methods in the Promo class:

1. _pick_random_bite is a helper (_ here means "private", not to be called directly on an instance) that picks a randomly available Bite (use the random module).
   
   When no more Bites are available raise the provided NoBitesAvailable custom exception.
   
   
2. new_bite should use this helper and update self.bites_done (it keeps state, the reason we used a class here).

See also the tests. We hope you learn a thing or two. Enjoy!

Update 4th of Nov 2020: we changed this Bite slightly to localize the Bite constants (thanks Kenneth). No worries: it's all done in the __init__ constructor for you, you don't have to change anything there).