gh-109861: Fix a doctest in Lib/_pydecimal.py which was failing when running its doctests directly (GH-125479)

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
(cherry picked from commit 5965f08)

Co-authored-by: Mark Byrne <31762852+mbyrnepr2@users.noreply.github.com>
Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>

Author: 2 people

Lib/_pydecimal.py

@@ -1821,9 +1821,10 @@ def __round__(self, n=None):
         Decimal('123.46')
         >>> round(Decimal('123.456'), -2)
         Decimal('1E+2')
-        >>> round(Decimal('-Infinity'), 37)
+        >>> with localcontext(ExtendedContext):
+        ...     round(Decimal('-Infinity'), 37)
+        ...     round(Decimal('sNaN123'), 0)
         Decimal('NaN')
-        >>> round(Decimal('sNaN123'), 0)
         Decimal('NaN123')
 
         """