Remove setuptools dependency

[vphilippon]

Setuptools is still required for pip-sync if pip needs it,
but pip-tools code itself doesn't require it anymore.
A pip-compile can be completed without setuptools.

Changelog-friendly one-liner: Removed the hard dependency on setuptools.

Contributor checklist

• Provided the tests for the changes
• Requested (or received) a review from another contributor
• Gave a clear one-line description in the PR (that the maintainers can add to CHANGELOG.md afterwards).

[davidovich]

LGTM