feat(pipenv): better python constraints checking

[rarkins]

Changes

Improves Pipenv Python constraint checking:

• Check Pipfile first (in preparation for updating this as a dependency)
• Check .python-version last

Context

Useful for Runinstall

Documentation (please check one with an [x])

• [ ] I have updated the documentation, or
• [x] No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• [ ] Code inspection only, or
• [ ] Newly added/modified unit tests, or
• [ ] No unit tests but ran on a real repository, or
• [x] Both unit tests + ran on a real repository