encoding: use double backslash

In Python 2, '\u' == '\\u'. However, in Python 3, '\u' results in:

SyntaxError: (unicode error) 'unicodeescape' codec can't decode bytes in
position 0-1: truncated \uXXXX escape

The minor change in this patch allows Python 3 to ast parse encoding.py.

mercurial/encoding.py

@@ -407,7 +407,7 b' def jsonescape(s):'
 
     if not _jsonmap:
         for x in xrange(32):
-            _jsonmap[chr(x)] = "\u%04x" %x
+            _jsonmap[chr(x)] = "\\u%04x" % x
         for x in xrange(32, 256):
             c = chr(x)
             _jsonmap[c] = c