Comment 2 for bug 1852356

Some evidence that this change is needed is that with it, the lower-constraints job fails:

Collecting psycopg2==2.6.2
  Downloading http://mirror.regionone.fortnebula.opendev.org/pypifiles/packages/7b/a8/dc2d50a6f37c157459cd18bab381c8e6134b9381b50fbe969997b2ae7dbc/psycopg2-2.6.2.tar.gz (376kB)
    ERROR: Command errored out with exit status 1:
     command: /home/zuul/src/opendev.org/openstack/glance/.tox/lower-constraints/bin/python3 -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-5w8p_8bb/psycopg2/setup.py'"'"'; __file__='"'"'/tmp/pip-install-5w8p_8bb/psycopg2/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /tmp/pip-install-5w8p_8bb/psycopg2/pip-egg-info
         cwd: /tmp/pip-install-5w8p_8bb/psycopg2/
[snip]
    Error: could not determine PostgreSQL version from '10.10'
    ----------------------------------------
ERROR: Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.

This error happens with versions of psycopg < 2.7 (see, for example, [0]). So the fact that we haven't seen it before illustrates the problem with our current tox configuration where the u-c are added as part of the install_command instead of in the deps.

[0] https://help.heroku.com/BFIHK81D/why-do-i-get-error-could-not-determine-postgresql-version-from-10-x-when-building-psycopg2