Update pipenv to 2020.11.15 and unpin pip

[edmorley]

Previously the buildpack used pipenv 2018.5.18, which in addition to being out of date doesn't support newer pip, meaning that apps using pipenv had to be pinned to a much older version of pip.

For apps using pipenv, the buildpack now installs pipenv 2020.11.15 and no longer overrides the pip version compared to non-pipenv installs, meaning pip 20.1.1 is now used instead of pip 9.0.2. (The pip version is still pinned, but to the reasonably new pip version used by all other non-pipenv builds.)

Changes:
https://github.com/pypa/pipenv/blob/master/CHANGELOG.rst#20201115-2020-11-15
pypa/pipenv@v2018.05.18...v2020.11.15

This is particularly important since cryptography v3.4 (released 2021-02-07) requires at least pip 19.x, otherwise pip is unable to use its newer style abi3 wheels, and so falls back to building the source distribution. This causes the install to fail, since building cryptography from the sdist now requires Rust, which is not present in the Heroku stack image. As seen in:

• https://heroku.support/960104
• https://heroku.support/960598
• https://heroku.support/961123

For more info on the cryptography issue, see:
https://cryptography.io/en/latest/faq.html#installing-cryptography-fails-with-error-can-not-find-rust-compiler
pyca/cryptography#5771
pyca/cryptography#5798

Fixes #979.
Fixes #987.
Fixes #1108.
Closes GUS-W-8054805.

[edmorley]

I'll release this to the buildpack registry tomorrow (which is the release that apps use with the buildpack URL of heroku/python). If anyone would like to try it out before then, you can switch to the buildpack URL of https://github.com/heroku/heroku-buildpack-python which uses the main branch from GitHub.

[edmorley]

This has now been released to the buildpack registry in #1170, so available via the heroku/python buildpack URL, and the following changelog entry published:
https://devcenter.heroku.com/changelog-items/2029