WIP Use pyproject.toml to specify build requirements

[alimanfoo]

[Description of PR]

TODO:

• Unit tests and/or doctests in docstrings
• tox -e py37 passes locally
• tox -e py27 passes locally
• Docstrings and API docs for any new/modified user-facing classes and functions
• Changes documented in docs/release.rst
• tox -e docs passes locally
• AppVeyor and Travis CI passes
• Test coverage to 100% (Coveralls passes)

[alimanfoo]

Including this here because then it allows developers to run setup.py directly, e.g., python setup.py build_ext --inplace.

[alimanfoo]

I'm running into issues getting this to work, e.g., tox does not pick up the build requirements in pyproject.toml, and appveyor (which doesn't use tox) has errors relating to setuptools-scm not creating version.py. I don't understand fully what's going on, so am going to back off for now and fall back to using setup_requires for py-cpuinfo, but will close and leave this PR as a record.

[alimanfoo]

xref pypa/pip#5407