bpo-29762: More use "raise from None". (#569)

This hides unwanted implementation details from tracebacks.
diff --git a/Lib/json/scanner.py b/Lib/json/scanner.py
index 86426cd..c451eba 100644
--- a/Lib/json/scanner.py
+++ b/Lib/json/scanner.py
@@ -29,7 +29,7 @@
         try:
             nextchar = string[idx]
         except IndexError:
-            raise StopIteration(idx)
+            raise StopIteration(idx) from None
 
         if nextchar == '"':
             return parse_string(string, idx + 1, strict)