py3: pass a str to getpass.getpass()

Otherwise I see `password: b''` on the prompt.

# skip-blame because just prefix

Differential Revision: https://phab.mercurial-scm.org/D6041

mercurial/ui.py

@@ -1553,7 +1553,7 b' class ui(object):'
                         raise EOFError
                     return l.rstrip('\n')
                 else:
-                    return getpass.getpass('')
+                    return getpass.getpass(r'')
         except EOFError:
             raise error.ResponseExpected()