Django-local-settings

- Tweak package metadata in `pyproject.toml`; in particular, specify the
README file so it will show up on PyPI; fix description and add
keywords too

- Fixed specification of minimum Django version in `pyproject.toml`
- Added changes to CHANGELOG for 2.0a2

- Added support for env settings, which are settings that have their
values loaded from environment variables; if a `.env` file is present,
environment variables will be loaded from it automatically
- Changed secret settings so they can specify a default as long as it's
a callable; this is convenient while discouraging use of the same
secret value in different environments
- Made some other improvements to the initial 2.0 alpha release

- Dropped support for Python versions less than 3.6
- Dropped support for Django versions less than 2.2
- Moving forward, only maintained Python and Django will be supported
- Considered replace INI+JSON with TOML but found the latter has some
limitations

- Dropped *official* support for Python 3.3 and 3.4 since these versions are
end-of-life. Since we still support Python 2.7 (for now), there shouldn't be
any issues on 3.3 or 3.4 (since we won't be using any Python 3.5-and-up-only
features).
- Added support for Python 3.8.
- Added support for Django 2.2.

- Made settings file/section inheritance more intuitive. This change is
backward incompatible but is, I think, unlikely to cause issues in
most cases because previously the workaround for the unintuitive way
inheritance worked was to add redundant settings in extending
sections. See ba09782e0608a47fe53f24d30ea7750acb0c4b0b.