py3: use bytes instead of str in isinstance()

We use bytes everywhere.

Differential Revision: https://phab.mercurial-scm.org/D6070

mercurial/tags.py

@@ -548,7 +548,7 b' def tag(repo, names, node, message, loca'
 
 def _tag(repo, names, node, message, local, user, date, extra=None,
          editor=False):
-    if isinstance(names, str):
+    if isinstance(names, bytes):
         names = (names,)
 
     branches = repo.branchmap()