Log Message:
py-betamax: update to 0.8.1.

0.8.1 - 2018-03-13
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- Previous attempts to sanitize cassette names were incomplete.
  Sanitization has become more thorough which could have some affects on
  existing cassette files. **This may cause new cassettes to be generated.**

- Fix bug where there may be an exception raised in a
  ``betamax.exceptions.BetamaxError`` repr.