Get command-line doctest of Lib/decimal.py to work again.

If tested as '__main__' instead 'decimal', the tracebacks would
abbreviate 'decimal.Inexact' as 'Inexact', breaking the doctests.

(Reviewed by Antoine.)
diff --git a/Lib/decimal.py b/Lib/decimal.py
index 1fe938f..c387de6 100644
--- a/Lib/decimal.py
+++ b/Lib/decimal.py
@@ -6245,5 +6245,5 @@
 
 
 if __name__ == '__main__':
-    import doctest, sys
-    doctest.testmod(sys.modules[__name__])
+    import doctest, decimal
+    doctest.testmod(decimal)