Relaxing `mock` requirement

...since Travis CI since to fail because of this.

See [1] for details.

[1] https://travis-ci.org/JensRantil/pep257/builds/10260682
Relaxing `pytest` requirement

Because there seem to be a conflict with, I assume, an upstream package.

See [1] for details.

[1] https://travis-ci.org/JensRantil/pep257/jobs/10260712