py3: don't use str() to stringify pushloc

contrib/python3-whitelist

@@ -69,6 +69,7 b' test-debugbundle.t'
 test-debugextensions.t
 test-debugindexdot.t
 test-debugrename.t
+test-default-push.t
 test-diff-binary-file.t
 test-diff-change.t
 test-diff-copy-depth.t

mercurial/ui.py

@@ -1757,7 +1757,7 b' def pushurlpathoption(ui, path, value):'
                   'ignoring)\n') % path.name)
         u.fragment = None
 
-    return str(u)
+    return bytes(u)
 
 @pathsuboption('pushrev', 'pushrev')
 def pushrevpathoption(ui, path, value):