Turn repo in to a python package

[symroe]

This PR makes the design system installable via pip or other standard python packaging tools.

For testing before merge, use pip install git+https://github.com/DemocracyClub/design-system.git@python-package.

This will install a python package that can be imported:

import dc_design_system

The only thing this package does is to expose the directory it lives in:

>>> import dc_design_system
>>> dc_design_system.DC_SYSTEM_PATH
'/home/symroe/.envs/design_system/lib/python3.8/site-packages/dc_design_system'

This tells us that the system folder in this repo is at /home/symroe/.envs/design_system/lib/python3.8/site-packages/dc_design_system/system.

This path can be used by libsass as an include-path, meaning that, once installed you can write project level sass assuming the system is assessable.

e.g, you can do this:

@import 'partials/_button.scss';

TODO

• I've called the folder python to indicate that this is about python, assuming we might want to include other package managers in the future. Does this make sense? Should it be python-package or something?
• Do we want to think about the names of things, given we might be using the system in other contexts? For example, system/partials/_button.scss is less clear than dc_design_system/partials/_button.scss (or whatever). I can rename this in the python package of course.
• We should get a working version of this in an actual project before merging @VirginiaDooley let's pair on this.

[Heydon]

@symroe Let's rename to python-package, I think the name benefits from being more specific.