devel/py-pytest: Update to 4.3.1

• Add a number of optional test dependencies to un-skip more tests
• Use PYTHON_VER instead of PYTHON_REL to match conditionals to upstream spec.
• Move mock to a conditional (2.7) dependency (per upstream spec)

Changelog:

https://github.com/pytest-dev/pytest/blob/4.3.1/CHANGELOG.rst

PR: 235339
Submitted by: John W. O'Brien <john saltant com> (with changes)
Approved by: koobs (python, maintainer)