[Python APIView] Support for packages with only pyproject.toml

[lmazuel]

As we're planning to move away from setup.py and have only a pyproject.toml, APIView needs to be able to handle source code with no setup.py.

This PR can be used to look into what a pure pyproject.toml package looks like:
Azure/azure-sdk-for-python#39100

Note that the wheel will be exactly the same (since a wheel do not contain packaging information anymore), it's only the source (Github and sdist) that are different

[swathipil]

For reading pyproject.toml, try not to use if "pyproject.toml" vs. "setup.py", look into using these packages:

• setuptools
• build
• pip