tox -epip-missing-reqs fails due to missing debtcollector in requirements

[Roman Podoliaka]

tox -epip-missing-reqs fails on current master:

Missing requirements:
oslo_db/exception.py:46 dist=debtcollector module=debtcollector.removals
ERROR: InvocationError: '/home/rpodolyaka/src/oslo.db/.tox/pip-missing-reqs/bin/pip-missing-reqs -d --ignore-module=oslo_db* --ignore-module=pkg_resources --ignore-file=oslo_db/tests/* oslo_db'
___________________________________________________________________ summary ___________________________________________________________________
ERROR: pip-missing-reqs: commands failed

We need to add debtcollector to requirements.txt

[Roman Podoliaka (rpodolyaka)]

Fix merged: https://review.openstack.org/#/c/307367/

[Doug Hellmann (doug-hellmann)]

This issue was fixed in the openstack/oslo.db 4.8.0 release.