statsd: Make arguments keyword-only

We don't use positional arguments anywhere, except in the one
place where we accidentally passed namespace as host:
swh/devel/swh-journal!260

swh/core/statsd.py

@@ -193,6 +193,7 @@ class Statsd(object):
 
     def __init__(
         self,
+        *,
         host=None,
         port=None,
         max_buffer_size=50,