pvec: fix a `str` type conditional for py3

Differential Revision: https://phab.mercurial-scm.org/D7538

mercurial/pvec.py

@@ -182,7 +182,7 b' def ctxpvec(ctx):'
 
 class pvec(object):
     def __init__(self, hashorctx):
-        if isinstance(hashorctx, str):
+        if isinstance(hashorctx, bytes):
             self._bs = hashorctx
             self._depth, self._vec = _split(util.b85decode(hashorctx))
         else: