py3: byte-stringify literals in extension in test-bundle2-exchange.t

# skip-blame just some b''

tests/test-bundle2-exchange.t

@@ -936,11 +936,11 b' Test lazily acquiring the lock during un'
   > import os
   > from mercurial import extensions, bundle2, error
   > def checklock(orig, repo, *args, **kwargs):
-  >     if repo.svfs.lexists("lock"):
-  >         raise error.Abort("Lock should not be taken")
+  >     if repo.svfs.lexists(b"lock"):
+  >         raise error.Abort(b"Lock should not be taken")
   >     return orig(repo, *args, **kwargs)
   > def extsetup(ui):
-  >    extensions.wrapfunction(bundle2, 'processbundle', checklock)
+  >    extensions.wrapfunction(bundle2, b'processbundle', checklock)
   > EOF
 
   $ hg init lazylock