Prevent import of PyQt4 with an ImportError

src/calibre/startup.py

@@ -24,6 +24,18 @@ winutil = winutilerror = None
 if not _run_once:
     _run_once = True
 
+    # Prevent PyQt4 from being loaded
+    class PyQt4Ban(object):
+
+        def find_module(self, fullname, path=None):
+            if fullname.startswith('PyQt4'):
+                return self
+
+        def load_module(self, fullname):
+            raise ImportError('Importing PyQt4 is not allowed as calibre uses PyQt5')
+
+    sys.meta_path.insert(0, PyQt4Ban())
+
     #
     # Platform specific modules
     if iswindows: