ci: Also downgrade PyQt5

While there's no actual "bad" changes in Qt 5.15.1/.2 outside of
QtWebEngine, many of our version checks still check for the qtbase
rather than the qtwebengine version.

See #3785

misc/requirements/requirements-pyqt-5.15.0.txt

@@ -1,5 +1,5 @@
 # This file is automatically generated by scripts/dev/recompile_requirements.py
 
-PyQt5==5.15.2  # rq.filter: < 6
+PyQt5==5.15.0  # rq.filter: == 5.15.0
 PyQt5-sip==12.8.1
 PyQtWebEngine==5.15.0  # rq.filter: == 5.15.0

misc/requirements/requirements-pyqt-5.15.0.txt-raw

@@ -1,4 +1,4 @@
-#@ filter: PyQt5 < 6
+#@ filter: PyQt5 == 5.15.0
 #@ filter: PyQtWebEngine == 5.15.0
-PyQt5 >= 5.15, < 6
+PyQt5 == 5.15.0
 PyQtWebEngine == 5.15.0