Major update with some backward incompatibilities

5.0 (2022-XX-XX)

Backward incompatibilities:

• Removed all Python 2 support, and Python 2-isms.
• Removed all remaining previously deprecated functionality.
• Dropped support for Python < 3.7; add support for Python up to 3.11.
• Switched to the Apache License Version 2.0.
• Changed the way getitem and call syntax are used to access enum items, to be more aligned with Python 3's standard enum library. Now you use getitem to look up an enum value by attribute name, and you use the call syntax to look up an enum value by value. The one difference remaining is that with flufl.enum you can use getitem syntax to look up an enum value by providing an enum value (standard library enum does not support this mode).

Code and Documentation:

• Added @public where appropriate.
• Fixed some typos in the README.
• Added an API reference document.
• Updated dependencies.

Housekeeping:

• Source tree reorganization.
• The master branch is renamed to main.
• Updated copyright years.
• 100% coverage; mypy clean; i-blue-it
• Use pdm as the package manager, and switch to pyproject.toml.
• Switched to the pytest runner.

Closes #2 (closed)