Update the testing doc page from unittest to pytest

The contributor testing page contained examples of writing UnitTest
unit tests. The references and examples have been updated with pytest
information instead of UnitTest information. Therefore, new or existing,
contributors will now have a brief introduction to writing and running
the Freeseer pytest unit tests.

Fix #566
Related #420
Related #484
Update docs README

Updated Sphinx installation instructions in README since Sphinx is now
in dev-requirements.txt.

Related to #575

[skip ci]