Fixed #33099 -- Improved performance of import_string().

This improves performance of import_string() by avoiding multiple
imports for the same path.

Thanks Andrew Godwin and Keryn Knight for the implementation idea.

django/utils/module_loading.py

@@ -1,9 +1,17 @@
 import copy
 import os
+import sys
 from importlib import import_module
 from importlib.util import find_spec as importlib_find
 
 
+def cached_import(module_path, class_name):
+    modules = sys.modules
+    if module_path not in modules:
+        import_module(module_path)
+    return getattr(modules[module_path], class_name)
+
+
 def import_string(dotted_path):
     """
     Import a dotted module path and return the attribute/class designated by the
@@ -14,10 +22,8 @@ def import_string(dotted_path):
     except ValueError as err:
         raise ImportError("%s doesn't look like a module path" % dotted_path) from err
 
-    module = import_module(module_path)
-
     try:
-        return getattr(module, class_name)
+        return cached_import(module_path, class_name)
     except AttributeError as err:
         raise ImportError('Module "%s" does not define a "%s" attribute/class' % (
             module_path, class_name)