Add build-gpep517 pipeline for Python

[Dentrax]

This PR adds a new Python pipeline called: python/build-gpep517.

In case if upstream project does not provide neither pyproject.toml or setup.py, so python/build and python/build-wheel pipelines does not work as expected since both pipelines uses build module and depends on aforementioned files to run correctly. Invoking the python-build with -m gpep517 rather than -m build, resolves this issue.

[kaniini]

Why not just use gpep517 directly in python/build? As far as I can tell gpep517 does handle legacy setup.py scripts?

[Dentrax]

As far as I can tell gpep517 does handle legacy setup.py scripts?

Hmmm, how can we be sure that it won't break the existing as-is flow? I was thinking to invoke gpep517 module if there isn't any setup.py.

[Dentrax]

Any thoughts? @kaniini

Does it make sense to check pyproject.toml and setup.py files whether exist, if so use as-is, otherwise fallback to gpep517 way?

[Dentrax]

How should we proceed here?

cc @imjasonh