Base on the updated configparser from 3.2+

Some remarks:

- I changed the syntax for the undocumented `"%(here)s"` to `"${HERE}"` to be consistent with the rest of the interpolation.
- Note that there is also buildout-compatible interpolation between sections.
- Your environment variables were previously not typed, for instance `SERVER_PORT=6543` would still land as a string. Now it's fixed.
- `SettingsDict` is now largely redundant using `configparser`'s mapping protocol support.