CI GUI updater test failing due to older pip version lacking wheel support

[zenmonkeykstop]

Description

In CI, the updater-gui-test job is consistently failing with errors like:

[pipenv.exceptions.InstallError]: ['Missing build requirements in pyproject.toml for pyqt5==5.14.1 from https://files.pythonhosted.org/packages/3a/fb/eb51731f2dc7c22d8e1a63ba88fb702727b324c6352183a32f27f73b8116/PyQt5-5.14.1.tar.gz#sha256=2f230f2dbd767099de7a0cb915abdf0cbc3256a0b5bb910eb09b99117db7a65b (from -r /tmp/pipenv-mh_2cphm-requirements/pipenv-d2u_fldx-requirement.txt (line 1)).', "  This version of pip does not implement PEP 517 so it cannot build a wheel without 'setuptools' and 'wheel'.", 'Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-install-9sehixs6/pyqt5/']
ERROR: ERROR: Package installation failed...

(The pip version is pinned in .circleci/config.yml at 18.0 since PR #3853, with a note to back that change out at some point.)

Steps to Reproduce

Run CI against a branch

Expected Behavior

CI passes

Actual Behavior

CI updater-gui-test job fails.