Use pytest as runner for doctests.

[drvinceknight]

I think this technically means that our coverage would be reported based on both our unit tests and our doctests. I'm not sure what I think of that. Completely up to you if we want this @meatballs.

[coveralls]

Coverage remained the same at 100.0% when pulling 8243b66 on pytest-docs into f625509 on master.

[coveralls]

Coverage remained the same at 100.0% when pulling c088419 on pytest-docs into f625509 on master.

[coveralls]

Coverage remained the same at 100.0% when pulling 72cdbc3 on pytest-docs into f625509 on master.

[drvinceknight]

No, this is no good: 72cdbc3 was not reported as an error by travis.