extensions: use [0:1] slice on config path instead of [0]

This behaves the same in Python 2 and Python 3, even though the path
is a bytes.

mercurial/extensions.py

@@ -160,7 +160,7 b' def loadall(ui):'
     newindex = len(_order)
     for (name, path) in result:
         if path:
-            if path[0] == '!':
+            if path[0:1] == '!':
                 _disabledextensions[name] = path[1:]
                 continue
         try: