Assign copyright @ docs to project contributors

[webknjaz]

This change mentions the pip-tools contributors specifically in the copyright and it also drops the year per The Linux Foundation best practices recommendation.

Ref: https://www.linuxfoundation.org/blog/copyright-notices-in-open-source-software-projects/

Contributor checklist

• Provided the tests for the changes.
• Assure PR title is short, clear, and good to be included in the user-oriented changelog
• Assure one of these labels is present: backwards incompatible, feature, enhancement, deprecation, bug, dependency, docs or skip-changelog as they determine changelog listing.
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).