varlink: change sleep_duration to a float

In another test it is set to 0.1 which upsets mypy as it changes the
type from a float to an int.

Author: jelly

varlink/tests/test_orgexamplemore.py

@@ -88,7 +88,7 @@ def __init__(self, reason):
 
 @service.interface("org.example.more")
 class Example:
-    sleep_duration = 1
+    sleep_duration = 1.0
 
     def TestMore(self, n, _more=True, _server=None):
         try: