Use a collective copyright notice in LICENSE.txt

[boronine-bot]

Summary

Replace Copyright (c) 2015 Alexei Boronine with:

Copyright The HSLuv Python Contributors.

This follows the collective-notice convention described in the Linux Foundation's Copyright Notices in Open Source Software Projects. The article recommends wording such as “Copyright The XYZ Contributors.” to acknowledge contributors without maintaining individual names and year ranges. Contributors retain ownership of their respective contributions; this notice does not assign copyright to a new entity.

Only the first line of LICENSE.txt changes. The MIT permission and warranty terms, package author metadata, and existing file-specific notices are unchanged.

Permission to replace the existing notice

The linked article also explicitly says not to change or remove someone else's copyright notice without their express written permission. Alexei Boronine should confirm permission to replace his existing notice before this PR is merged.

Validation

• Verified that everything after the first line is byte-for-byte unchanged.
• Verified that only LICENSE.txt changed.
• git diff --check: passes.