Fix a doctest in Lib/_pydecimal.py which was failing when run

directly on the module via the command:
``./python.exe -m doctest Lib/_pydecimal.py``.

Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>

Author: mbyrnepr2

Lib/_pydecimal.py

@@ -1796,6 +1796,7 @@ def __round__(self, n=None):
         Decimal('123.46')
         >>> round(Decimal('123.456'), -2)
         Decimal('1E+2')
+        >>> setcontext(ExtendedContext)
         >>> round(Decimal('-Infinity'), 37)
         Decimal('NaN')
         >>> round(Decimal('sNaN123'), 0)