Libimagequant-integrations

Version 1.1.1 of libimagequant_integrations adds new `libimagequant_integrations.PyQt6` and `libimagequant_integrations.PySide6` modules! This is a fully backwards-compatible release.

- New module! `libimagequant_integrations.png`, integrating with PyPNG.
- Transparency is now handled properly, and tests have been added to ensure this. (This doesn't apply to Pillow, which doesn't really support transparent paletted images properly in the first place.)
- A few minor fixes and improvements.

Version 1.0.0 of libimagequant_integrations (the first release).