Fix test for Python 3.9

Python 3.9 has neither dummy_thread nor _dummy_thread anymore. Using _thread.allocate_lock passes all tests here.

src/aspectlib/test.py

@@ -29,7 +29,10 @@
 try:
     from dummy_thread import allocate_lock
 except ImportError:
-    from _dummy_thread import allocate_lock
+    try:
+        from _dummy_thread import allocate_lock
+    except ImportError:
+        from _thread import allocate_lock
 try:
     from collections import OrderedDict
 except ImportError: