python-build: add support for pyproject.toml files

A new PEP 517 (https://www.python.org/dev/peps/pep-0517/) has defined that
Python packages can be shipped without any `setup.py` file, and that a
`pyproject.toml` file is sufficient.

A `setup.py` shim layer is suggested as a method for running the build.

For these cases, we will add a support in the OpenWrt build-system to
provide the default `setup.py` shim layer in case this file does not exist,
but there is a `pyproject.toml` file.

We also seem to need to tweak the shim layer with the PKG_VERSION,
otherwise the detected version is 0.0.0.
We will need to see if this will be fixed later in setuptools{-scm}.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>
(cherry picked from commit 61f202c0170785addbbc449e4de61cc5886f0833)

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