Imperfect

* Add `.index` method used by the mapping methods on both `ConfigFile` and
`ConfigSection`. It returns the first match, like lists do, and is
case-insensitive by default.
* Add item deletion, e.g. `del conf["sectionname"]`
* Update skel, no longer produce `py2.py3` wheels and put `setup_requires` in
`pyproject.toml`
* Bug fix: Include missing requirements.txt, reference from `make setup`
* Bug fix: when using `set_value` with empty string, don't forget the newline.

* Add `ConfigFile.text` property
* Sample code now uses `moreorless`
* Better types on `.build`
* Bug fix: when using `set_value` with multiline strings, don't include a
trailing whitespace

* Bug fix: whem using `set_value` and it adds a section to the end of the file,
include a blank line before.

* When using `set_value` with multiline strings, include hanging indent instead
of requiring the caller to do so.

* Improve CI config
* Include tests and py.typed
* Add `.set_value`

* More directly match configparser behavior, coverage to 90%