Work around PEP-517 pip build system installing dev dependencies

[izakp]

I started to get errors installing hokusai via pip install . in a Docker container due to unavailable gcc tools and realized introducing the pyproject.toml file caused pip to use poetry as its build system which then installed dev dependencies.

Pip introduced support for PEP-517 -- A build-system independent format for source trees in version 10 which respects the pyproject.toml file and support for the build-backend in version 19. These errors only manifest while running on pip version 19 or 20 so followed the lead of this PR to default to the legacy build backend (i.e. setuptools).

Here are similar issues:

• "conflict with the backend dependencies" error running pip install in editable mode with pyproject.toml / PEP-517 enabled project pypa/pip#6314
• "conflict with the backend dependencies" with pip 19 python-poetry/poetry#826

pip install -e . / pip install . now work as expected and only install dependencies specified in setup.py. It may be nice in the future to use poetry as a production build backend but will require further investigation.

[izakp]

Going to merge this to ensure pip installs from master work properly