switch to setuptools_scm

[nitzmahone]

• remove pbr usage entirely
• switch to PyPA-recommended package layout (Python packages under root src/ dir)
• remove legacy setup.py in favor of PEP517 pyproject.toml
• still using setup.cfg for most metadata until setuptools support for it in pyproject.toml stabilizes further
• pin min/max PEP518 build deps for setuptools and setuptools_scm to avoid future packaging metadata deprecation/removal issues
• configure export-subst ref/metadata smuggling in archives for setuptools_scm when not installing from a git checkout or sdist (eg downstream packagers installing directly from GH ref tarballs)

[sivel]

There is some pbr stuff left in docs/ that needs to be removed, it isn't used or required at all based on my research for runner. Although part of it is modifying docs/requirements.in which is used to generate (manually, no in repo tooling) the docs/requirements.txt file. And looks like with runner sphinx-ansible-theme is missing from requirements.in.

It's possible the docs/requirements.txt can just be edited, without regenerating.