nose2pytest: Upgrade Python testing from nose to pytest

[cclauss]

IMPORTANT

Fixes #92
Fixes #93
Fixes #95
Similar to xlcnd/isbntools#119

Make your pull request against the dev branch, please!

On follow-up of issue #95

Changes proposed in this pull request:

• Run https://pypi.org/project/nose2pytest (on Python 3.6!!) to upgrade Python testing from nose to pytest.
• I am unsure why pytest --ignore=isbnlib/test/test_editions.py --ignore=isbnlib/test/test_openl.py is required.
• After this is merged, there will be a todo to wire together pytest and cov.

@xlcnd

[xlcnd]

Thanks!