Hi everyone,

the attached patches switch python-build-system to a PEP 517-based build
system using python-pypa-build.

As discussed previously Python is currently in the process of opening up
for build systems other than setuptools using the API specified in PEP
517. python-pypa-build is a simple tool for building packages using that
new API. It supports setup.py-based builds as a fallback, if no
pyproject.toml is present.

One downside is that this tool is not self-contained and has a few
dependencies. Thus first I bootstrap setuptools using itself (possible
because it bundles all of its own dependencies), then build
python-pypa-build’s dependencies using setuptools (which is fortunately
still possible) and then combine everything into a
python-toolchain(-for-build), which is then used by the build-process.

I can successfully build packages like python-pypa-build and
python-pytest and python-pep517-bootstrap. The latter is using flit as
its build backend. But other packages currently fail because I removed
some arguments.

Cheers,
Lars