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Bingo! doctest sets the __builtins__._ variable,
and the last doctest in decimal.py (in lexicographic order) is
Decimal.__round__, which ends with
 >>> round(Decimal('sNaN123'), 0)
 Decimal('NaN123')
This is the value you see in Benjamin's output.
Here is a trivial patch:
Index: Lib/test/regrtest.py
===================================================================
--- Lib/test/regrtest.py (revision 65283)
+++ Lib/test/regrtest.py (working copy)
@@ -647,6 +647,8 @@
 def cleanup_test_droppings(testname, verbose):
 import shutil
+ __builtins__._ = None
+
 # Try to clean up junk commonly left behind. While tests shouldn't
leave
 # any files or directories behind, when a test fails that can be
tedious
 # for it to arrange. The consequences can be especially nasty on
Windows,