Support pipenv

[dsayling]

With https://github.com/CircleCI-Public/cimg-python/issues/39 comes virtualenv and pipenv in the image.

Suggested orb features:

• pipenv install and the arguments it allows
• pipenv also supports environment variables for a lot of the command flags - this may simplify parameterization
• pipenv shell
  • the shell activates and loads another shell - what are the implications of this from a noninteractive session? what environment variables are loaded/changed beyond the python paths?
• pipenv run
  • this should be the standard way
• save/load virtualenv cache
  • pipenv --venv will return the path to the virtualenv it created - The venv's path is not always the same IIRC
• install pipenv via pip
  • legacy images or older versions will still want to install pipenv to use this functionality in the orb
• pipenv generate requirements.txt
• pipenv check - checks security vulnerabilities and PEP 508
  • This requires the https://pyup.io/ key envvar PIPENV_PYUP_API_KEY set securely

Some inspiration from this community orb: https://circleci.com/orbs/registry/orb/frameio/pipenv

Some devs may want virtualenv support only - without pipenv, but that support can be added independently.

[wyardley]

Note that I think pipenv check may require a commercial API key to work at all

I tried using it and ended up using safety check instead, but maybe I'm missing something on getting pipenv check to work without the paid token.