Pipenv and pip version can be set through env var

[poulainv]

Add the ability to override PIP_VERSION & PIPENV_VERSION
I deployed a review app using it (and successfully added ddtrace) — there

how

• PIPENV_VERSION can not be used as env var see there
• Heroku configuration variables are available in files see there

[ElPicador]

Nice 👍

Thanks for trying this!

[Thomas-Mollard]

Thanks a lot for taking care of this!

[timmygee]

This fix saved my hide. By default it uses a much newer version of pip which means modules like cryptography actually build. I didn't even need to change the pipenv version but it's great to have that there as an option. Heroku really needs to support modern python better. The current buildpack version of pip (9.0.2) just does not cut it anymore.

@edmorley please merge upstream!

[poulainv]

Thanks @timmygee, it became required on our side as well. A few other PR have been already submitted, never merged. See @edmorley's comment about it.

[edmorley]

Hi! The pipenv version has just been updated in heroku#1169. Sorry for the delay! There's some more context in:
heroku#987 (comment)

Re allowing the pipenv/pip versions to be configured via environment variables, this is something I wouldn't recommend - I've added some additional notes on this to:
heroku#1094 (comment)