Django-local-settings

In progress...

- Dropped official support for Python 3.6 since it will be EOL at the
end of December 2021
- Added support for Python 3.10
- Split out the `jsonish` package into a new `jsun` package, which is
installable from PyPI

- Fixed return type of `jsonish.scanner.Scanner.make_scan_method()`.
- Add changelog for 2.0a7

- Improved JSON decoder.
- Changed `INIJSONStrategy` to use a single JSON decoder instance rather
than repeatedly recreating it for *every* setting.
- Improved interpolation:
- Empty `{{ }}` groups will now raise an error
- Unmatched closing `}}` will now raise an error

- Improved support for additional JSON features by writing a whole new
parser (because why not). This fixes the issue note in 2.0a5 where
the additional date and number formats couldn't be used in nested
structures. It also adds support for inline comments (// style), adds
customization hooks, and allows all the extra features to be disabled
so that only standard JSON will be accepted.

- Added support for trailing commas in list and dict settings. This
makes for consistency, nicer diffs, easier copy/paste from Python
code, etc.
- Added support for additional number formats in settings. Any valid
Python `int` or `float` will now work.
- Added support for literal date and time settings, similar to TOML.
- Added initial Sphinx doc setup. Still need to actually write docs...