Pin pip to 18.x

[ob-stripe]

r? @brandur-stripe
cc @remi-stripe

Temporary workaround to make the builds green again.

The issue is caused by a combination of unfortunate factors:

• we have a pyproject.toml file, which is a sort of new Python thing (PEP 518) to replace older ways of specifying dependencies. But we only use it to configure Black (the code formatter)
• recent pip versions will no longer let you install a package in editable mode if there is a pyproject.toml without a build-backend (cf. Show a nice error if editable mode is attempted with a pyproject.toml source tree pypa/pip#6331). You can disable this behavior by passing --no-use-pep517, but...
• pipenv doesn't have a way to do this (cf. A way to specify build-dependencies pypa/pipenv#3651)

For now, we can just downgrade pip to 18.x to avoid the issue.

Mid/long term, we should probably:

• fully replace setup.py with pyproject.toml
• get rid of pipenv

[brandur-stripe]

Thanks for the fix!

• fully replace setup.py with pyproject.toml
• get rid of pipenv

+1.