pvec: fix overlooked chr() call

Caught by pytype.

Differential Revision: https://phab.mercurial-scm.org/D7261

mercurial/pvec.py

@@ -76,7 +76,7 b' def _bin(bs):'
 def _str(v, l):
     bs = b""
     for p in pycompat.xrange(l):
-        bs = chr(v & 255) + bs
+        bs = pycompat.bytechr(v & 255) + bs
         v >>= 8
     return bs