lfs: use str for the open() mode when opening a blob for py3

The other fix for this was to leave the mode as bytes, and import
`pycompat.open()` like a bunch of other modules do.  But I think it's confusing
to still use bytes at the python boundary, and obviously error prone.  Grepping
for ` open\(.+, ['"][a-z]+['"]\)` and ` open\(.+, b['"][a-z]+['"]\)` outside of
`tests`, there are 51 and 87 uses respectively, so it's not like this is a rare
direct usage.

Differential Revision: https://phab.mercurial-scm.org/D8099

hgext/lfs/blobstore.py

@@ -128,7 +128,7 class local(object):
     def open(self, oid):
         """Open a read-only file descriptor to the named blob, in either the
         usercache or the local store."""
-        return open(self.path(oid), b'rb')
+        return open(self.path(oid), 'rb')
 
     def path(self, oid):
         """Build the path for the given blob ``oid``.