typing: fix forward reference in _Tlocalstr type bound

AFAIK, a quoted string in type position is translated to its ForwardRef.

  https://github.com/python/typing/issues/34

This fixes the following error:

  File "mercurial/encoding.py", line 38, in <module>: Name 'localstr' is not
  defined [name-error]

mercurial/encoding.py

@@ -35,7 +35,7 b' if not globals(): # hide this from non-'
     for t in (Any, Callable, List, Text, Type, Union):
         assert t
 
-    _Tlocalstr = TypeVar('_Tlocalstr', bound=localstr)
+    _Tlocalstr = TypeVar('_Tlocalstr', bound='localstr')
 
 charencode = policy.importmod('charencode')