python: pip-tools: 4.2.0 -> 4.3.0

There is a new pip-tools release. A commit [removing the test dependency
on six] is included in this release, so several tests that now pass are
being re-enabled in the check phase. The six-specific tests that still
don't pass require network access and remain disabled. Finally, drop the
dependency on first, which has been dropped by pip-tools. The full
changelog is available at https://github.com/jazzband/pip-tools/blob/f28d1a7d25c8a490071b01e48b3c1b393ada2e23/CHANGELOG.md#430-2019-11-25.

[removing the test dependency on six]: https://github.com/jazzband/pip-tools/pull/939