py3: add b'' prefix in tests/test-revlog-v2.t

After this, the test works on Python 3. There is some extra return value by
open().write() I think which needs to be handled.

# skip-blame because just b'' prefix

Differential Revision: https://phab.mercurial-scm.org/D2509

tests/test-revlog-v2.t

@@ -29,7 +29,7 b' Can create and open repo with revlog v2 '
 Unknown flags to revlog are rejected
 
   >>> with open('.hg/store/00changelog.i', 'wb') as fh:
-  ...     fh.write('\x00\x04\xde\xad')
+  ...     fh.write(b'\x00\x04\xde\xad')
 
   $ hg log
   abort: unknown flags (0x04) in version 57005 revlog 00changelog.i!