TT#14008 don't use f'' strings in python tests

Older Debians use an older Python that doesn't support these strings

Change-Id: Ib097fc5d33539bfa5259a10f83dccf4290d6eedf

t/auto-daemon-tests-websocket.py

@@ -23,7 +23,7 @@ async def get_ws(cls, proto):
 
     if sys.version_info >= (3, 10) and float(__version__) <= 9.1:
         python_v = python_version()
-        msg = f"python3-websocket {__version__} unsupported in {python_v}"
+        msg = "python3-websocket {} unsupported in {}".format(__version__, python_v)
         raise unittest.SkipTest(msg)
     for _ in range(1, 300):
         try: