faster PEP517 builds

[dimbleby]

minor simplifications and tweaks to pep517 builds

• update version of build used during metadata extraction
• if pep517 metadata extraction fails, don't try fallback to setup.py egg_info
  • eggs are long deprecated
  • using setup.py directly from the command line is deprecated
  • if pep517 builds fail then poetry will later refuse to install the package in any case
• no need to install setuptools or wheel in the build environments, if they are needed as build requirements then they will be installed as build requirements
• need pip in the build environment for metadata extraction because we run venv.run_pip(), but don't need it in the build environment during final build

[abn]

For context around the deprecation of the python setup.py <cmd>, future visitors can check https://packaging.python.org/en/latest/discussions/setup-py-deprecated/

[abn]

no need to install setuptools or wheel in the build environments, if they are needed as build requirements then they will be installed as build requirements

I am not sure if we can rely on this assumption yet. However, I suspect we will have to pull the trigger on this one at some point anyway. Now might be as good a time as any.

[github-actions]

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