Fix package keywords

Begin development of version 0.5

Release version 0.4

Tweak package manifest

Add type hints

Clarify variable names

Remove unnecessary custom constructor from `MessageFormatError`

Tweak changelog

Turn namedtuples `Message`, `PriorityValue` into dataclasses

Remove support for Python 3.6

Primary reason is to be able to use dataclasses.

Provide public `parse` function in parser module, "hide" parser class

Turn `PriorityValue` class factory method into function

Use new-style classes

Begin development of version 0.4

Release version 0.3

Clarify changelog

Format code (more) according to Black

Move package configuration to `setup.cfg`

Move to a `src/` project layout

Update copyright notes for 2021