Hands on: Write implementation for `pyanno_voting.labels_frequency`

[pberkes]

In the package pyanno, located at [1] , there is an empty function called labels_frequency [2]

1. Write a test for the function, run it until it fails for the right reason (i.e., because the current implementation does nothing and returns None). The example in the docstring can be used as basis for the test.
2. Write an implementation that makes the test pass

[1] https://github.com/ASPP/2019-camerino-testing-debugging/tree/master/hands_on/pyanno_voting
[2] https://github.com/ASPP/2019-camerino-testing-debugging/blob/master/hands_on/pyanno_voting/pyanno/voting.py#L83