Backwards incompatible changes:

* Drop support for Python 3.8. Thanks, Rob!

Fixes and features:

* Add support for Python 3.13. (#260) Thanks, Rob!

* Add support for underscore as first character in variable names in env files.
  (#263)

* Add ``ChoiceOf`` parser for enforcing configuration values belong in
  specified value domain. (#253)

* Fix ``autocomponentconfig`` to support components with no options. (#244)

* Add ``allow_empty`` option to ``ListOf`` parser that lets you specify whether
  empty strings are a configuration error or not. (#268)