bpo-39524: Fixed doc-string in ast._pad_whitespace (GH-18340)

Lib/ast.py

@@ -302,7 +302,7 @@ def _splitlines_no_ff(source):
 
 
 def _pad_whitespace(source):
-    """Replace all chars except '\f\t' in a line with spaces."""
+    r"""Replace all chars except '\f\t' in a line with spaces."""
     result = ''
     for c in source:
         if c in '\f\t':