Pin to oldest supported PyQt on minver CI instance.

This will be relevant once we also include PyQt6 support, as details of
enum access have changed in PyQt 5.11.

PyQt is not actually a dependency so I don't think we can use the
constraints file (minver.txt) for that.

Author: anntzer

.github/workflows/tests.yml

@@ -25,6 +25,7 @@ jobs:
             os: ubuntu-16.04
             python-version: 3.7
             extra-requirements: '-c requirements/testing/minver.txt'
+            pyqt5-ver: '==5.8 sip==4.19.7'  # oldest versions with a Py3.7 wheel.
             delete-font-cache: true
             XVFB_RUN: xvfb-run -a
           - os: ubuntu-16.04
@@ -168,7 +169,7 @@ jobs:
             # Sept 2020) for either pyqt5 (there are only wheels for 10.13+) or
             # pyside2 (the latest version (5.13.2) with 10.12 wheels has a
             # fatal to us bug, it was fixed in 5.14.0 which has 10.13 wheels)
-             python -mpip install --upgrade pyqt5 &&
+             python -mpip install --upgrade pyqt5${{ matrix.pyqt5-ver }} &&
                python -c 'import PyQt5.QtCore' &&
                echo 'PyQt5 is available' ||
                echo 'PyQt5 is not available'