Unpin constraint on `packaging`

[di]

This was constrained in #12664 due to googleapis/python-bigquery#1435 which is now resolved:

warehouse/requirements/deploy.in

Line 4 in 186180c

packaging<22.0

We attempted to unconstrain it in #12871, however due to pypa/packaging#407, we can't migrate to the latest version until pypa/packaging#669 is resolved.

[di]

This now needs a release of packaging which includes pypa/packaging#669, a release of https://github.com/di/packaging_legacy, and an update here to use that dependency.