[python] Block imports of PyQt4 modules

Prevents crashes when PyQt4 modules are imported in QGIS 3.x.
This instantly segfaults QGIS. Throwing an exception makes it
easier to identify the cause as a faulty plugin, and shows
exactly where the bad import is located.

Author: nyalldawson

python/utils.py

@@ -659,9 +659,16 @@ def __exit__(self, exc_type, exc_val, exc_tb):
 
 
 def _import(name, globals={}, locals={}, fromlist=[], level=None):
-    """ wrapper around builtin import that keeps track of loaded plugin modules """
+    """
+    Wrapper around builtin import that keeps track of loaded plugin modules and blocks
+    certain unsafe imports
+    """
     if level is None:
         level = 0
+
+    if 'PyQt4' in name:
+        raise ImportError('Cannot import PyQt4 classes - this would cause a crash!')
+
     mod = _builtin_import(name, globals, locals, fromlist, level)
 
     if mod and '__file__' in mod.__dict__: