In Python 2, `subprocess.check_output` returns a string. In Python 3, it
returns a byte string unless you provide `text=True`.
This makes sure all calls have that option for compatibility.
Also, my editor was auto-formatting these with [Black][0], which I
didn't prevent, so now these files are formatted a little better.
[0]: https://github.com/psf/black
- Avoid accessing `build` in FeatureFlags.swift
- Make it private so others aren't tempted
- Now that build is private, drop the named build variants. It's cute,
but ultimately not necessary
