Betty 0.4.0a3

What's Changed

• Add a plugin API by @bartfeenstra in #1554
• Make extension classes final by @bartfeenstra in #1669
• Make Extension API classes abstract by @bartfeenstra in #1670
• Make Cache API classes abstract and final by @bartfeenstra in #1671
• Make Locale API classes abstract and final by @bartfeenstra in #1673
• Make Plugin API classes abstract and final by @bartfeenstra in #1674
• Make Dispatch API classes abstract and final by @bartfeenstra in #1675
• Make core component classes abstract and final by @bartfeenstra in #1672
• Make Render API classes abstract and final by @bartfeenstra in #1676
• Make Requirement API classes abstract and final by @bartfeenstra in #1677
• Make Serde API classes abstract and final by @bartfeenstra in #1680
• Make Concurrent API classes abstract and final by @bartfeenstra in #1681
• Make URL API classes abstract and final by @bartfeenstra in #1679
• Make Event Type API classes abstract and final by @bartfeenstra in #1682
• Make Serve API classes abstract and final by @bartfeenstra in #1678
• Make Presence Role API classes abstract and final by @bartfeenstra in #1683
• Make Configuration API classes abstract and final by @bartfeenstra in #1684
• Make dispatcher event classes abstract by @bartfeenstra in #1685
• Make Deriver API classes abstract and final by @bartfeenstra in #1687
• Remove RequirementCollection.add() by @bartfeenstra in #1686
• Make test classes abstract and final by @bartfeenstra in #1689
• Make entity classes abstract and final by @bartfeenstra in #1688
• Refactor extensions into plugins by @bartfeenstra in #1666
• Refactor URL generators so their normal operations do not rely on large numbers of exceptions being raised by @bartfeenstra in #1691
• Remove FileBasedConfiguration by @bartfeenstra in #1690
• Refactor CLI commands into plugins by @bartfeenstra in #1661
• Separate the Configuration API into submodules by @bartfeenstra in #1692
• Add an image manipulation API by @bartfeenstra in #1693
• Fix a typo by @bartfeenstra in #1695
• Remove betty.fs.iterfiles() by @bartfeenstra in #1697
• Refactor entity types into plugins by @bartfeenstra in #1694
• Refactor presence roles into plugins by @bartfeenstra in #1698
• Refactor event types into plugins by @bartfeenstra in #1560

Full Changelog: 0.4.0a2...0.4.0a3