Good catch Neal!

I completely forgot about pyo files and the tests are usually not run with -O. The modified code checks for *.py?

Lib/test/test_import.py

@@ -213,7 +213,8 @@ class ImportTest(unittest.TestCase):
             os.remove(source)
             del sys.modules[TESTFN]
             mod = __import__(TESTFN)
-            self.failUnless(mod.__file__.endswith('.pyc'))
+            ext = mod.__file__[-4:]
+            self.failUnless(ext in ('.pyc', '.pyo'), ext)
         finally:
             sys.path.pop(0)
             remove_files(TESTFN)

Python/import.c

@@ -982,7 +982,8 @@ get_sourcefile(const char *file)
 	}
 
 	len = strlen(file);
-	if (len > MAXPATHLEN || PyOS_stricmp(&file[len-4], ".pyc") != 0) {
+        /* match '*.py?' */
+	if (len > MAXPATHLEN || PyOS_strnicmp(&file[len-4], ".py", 3) != 0) {
 		return PyUnicode_DecodeFSDefault(file);
 	}