The testsetup in Doc/library/decimal.rst is not enough for isolating the tests in respect to the other rst files.  Currently we have the following testsetup, without a testcleanup:

 .. testsetup:: *
 
    import decimal
    import math
    from decimal import *
    # make sure each group gets a fresh context
    setcontext(Context())

Without a testcleanup, the changes on the context will affect the other files that use the context (like Doc/library/statistics.rst).  We should better isolate the tests adding also a testcleanup:

.. testcleanup:: *

   # make sure other tests (outside this file) get a fresh context
   setcontext(Context())

I am opening a PR.