chore: Add property testing using hypothesis

[qTipTip]

Adding property testing using hypothesis. This PR makes sure that palettes produced by extract_colors maintain some invariants. The property tests also uncovered an issue with extracting colors from images with less pixels than requested colors, when resize was set to False. This has been fixed.

• add hypothesis as a dev dependency
• add invariant tests
• break early if box cannot be split in median cut
• clamp number of colors to the number of pixels to avoid degenerate cases
• add a note on frequencies in Palette
• Add a list of invariants that extract_colors respect
• update changelog