setuptools3: check for a PEP517 build system selection

setuptools3: check for a PEP517 build system selection

Some recipes use setuptools3 but should be using a PEP517-compliant
backend, be it the setuptools itself via python_setuptools_build_meta or
an alternative backend such as flit/hatch/poetry.

As we can't currently assume Python 3.11 on the build host we need to
parse the pyproject.toml manually, but this should be sufficient for the
limited parsing needed.

This task emits a QA error if a build backend is set, and can be ignored
by using INSANE_SKIP if needed.

[ YOCTO #14736 ]

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>