mavexpression.py: cope with imp module being removed in 3.12

mavexpression.py

@@ -19,8 +19,28 @@
 if home is not None:
     extra = os.path.join(home, '.pymavlink', 'mavextra.py')
     if os.path.exists(extra):
-        import imp
-        mavuser = imp.load_source('pymavlink.mavuser', extra)
+        try:
+            import imp
+            mavuser = imp.load_source('pymavlink.mavuser', extra)
+        except ModuleNotFoundError:
+            # "imp" is removed in Python 3.12.  Try to use importlib instead:
+            import sys
+            # from: https://docs.python.org/dev/whatsnew/3.12.html#removed
+            import importlib.util
+            import importlib.machinery
+
+            def load_source(modname, filename):
+                loader = importlib.machinery.SourceFileLoader(modname, filename)
+                spec = importlib.util.spec_from_file_location(modname, filename, loader=loader)
+                module = importlib.util.module_from_spec(spec)
+                # The module is always executed and not cached in sys.modules.
+                # Uncomment the following line to cache the module.
+                sys.modules[module.__name__] = module
+                loader.exec_module(module)
+                return module
+
+            load_source('pymavlink.mavuser', extra)
+
         from pymavlink.mavuser import *
 
 def evaluate_expression(expression, vars, nocondition=False):