Support pip10

[CaseyFaist]

We are currently on 9.0.2 - let's update

[cjolowicz]

Yes, please. This should make it possible to install projects using poetry, or any other build backend, without requiring other changes to the buildpack:

pip install .

From the pip reference guide:

As of version 10.0, pip supports projects declaring dependencies that are required at install time using a pyproject.toml file, in the form described in PEP 518. When building a project, pip will install the required dependencies locally, and make them available to the build process. Furthermore, from version 19.0 onwards, pip supports projects specifying the build backend they use in pyproject.toml, in the form described in PEP 517.