Fix typo in test (#456)

Author: kianmeng

tests/test_sourcecode.py

@@ -43,7 +43,7 @@ def test_mypy(self):
         try:
             import mypy  # NoQA
         except ImportError:
-            raise unittest.SkipTest('mypy moudule is missing')
+            raise unittest.SkipTest('mypy module is missing')
 
         try:
             subprocess.run(