statprof: correctly always pass a str as the thread name

Caught by pytype.

Differential Revision: https://phab.mercurial-scm.org/D7259

mercurial/statprof.py

@@ -352,7 +352,7 b" def start(mechanism=b'thread', track=b'c"
             frame = inspect.currentframe()
             tid = [k for k, f in sys._current_frames().items() if f == frame][0]
             state.thread = threading.Thread(
-                target=samplerthread, args=(tid,), name=b"samplerthread"
+                target=samplerthread, args=(tid,), name="samplerthread"
             )
             state.thread.start()