CI tests: Pin pytest until we can update pytest-qt

[john-dupuy]

Address #62

• Looks like pytest-qt is hitting an issue similar to Add py as dependency pytest-dev/pytest-html#555
• We cannot update it until we remove qgis 3.10 support. So, let's pin pytest for now.

Also, disable running tests on the release-3_22 tag, it looks like that image is having problems (no module called qgis). I just replaced it with the release-3_26 tag.