Change license headers to REUSE headers

**Is your feature request related to a problem? Please describe.**
I want to make my open source library, which is dependent on kelp, REUSE compliant. I vendor kelp and currently have to add the REUSE headers each time I update kelp.

**Describe the solution you'd like**
Kelp should adopt the REUSE format for the copyright notice.

**Describe alternatives you've considered**
Keeping a hacky script.

**Additional context**
[REUSE](https://reuse.software/)

No files, tests, or entry points are named; review the repository's existing license headers and the REUSE specification first. Done means kelp uses REUSE-formatted copyright notices throughout the vendored library so downstream users no longer need a script to add them.

Written by the indexing model from the issue text.