Update copyright notes for 2025
Update ignore pattern for virtual environment
Remove `setup.py`
It is no longer required for `pip install -e .` to work as of pip v21.2.
Move release dates to changelog headings
Remove redundant "Version" prefix from changelog headings
Begin development of version 0.5
Remove unnecessary custom constructor from `MessageFormatError`
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