Enable pytest rewriting in test helper functions.

[QuLogic]

If you call a helper function from numcodecs.tests.common and it fails, pytest will show a generic assertion message. But with assertion rewriting enabled, it will shows its full verbose output (i.e., expanding left and write values, showing local variables, etc.)

TODO:

• Unit tests and/or doctests in docstrings
• tox -e py37 passes locally
• tox -e py27 passes locally
• [N/A] Docstrings and API docs for any new/modified user-facing classes and functions
• Changes documented in docs/release.rst
• tox -e docs passes locally
• AppVeyor and Travis CI passes
• Test coverage to 100% (Coveralls passes)

[alimanfoo]

Cool, I didn't know about this, thank you!

Happy to merge once CI passes.

[jakirkham]

Thanks @QuLogic :)