Fix expected error message for python 3.10

python 3.10 adds some words to it's errors, so catch if our expected
are contained in the actual output

Signed-off-by: Arthur Zamarin <arthurzam@gmail.com>

cssutils/tests/basetest.py

@@ -126,7 +126,7 @@ def assertRaisesMsg(self, excClass, msg, callableObj, *args, **kwargs):
             if not msg:
                 # No message provided: any message is fine.
                 return
-            elif excMsg == msg:
+            elif msg in excMsg:
                 # Message provided, and we got the right message: passes.
                 return
             else: