Issue #12080: Fix a performance issue in Decimal._power_exact that causes some corner-case Decimal.__pow__ calls to take an unreasonably long time.

test_multibytecodec_support. Improve also error message of the
test_errorhandle() of TestBase.

test_multibytecodec_support. Improve also error message of the
test_errorhandle() of TestBase.

The previous markup hijacked the abstract-base-classes glossary entry,
which resulted in the HTML linking to collections.abc when defining the
generic ABC concept.  Now the glossary links to the abc module.

I’m afraid my docstring is not as clear as it should be, but I can’t
find a concise way of describing what the code does.  Suggestions for
improvements welcome.

Keep a timeout of 1 hour for buildbots.

Also promote notices from distutils doc to deprecation boxes.

This removes the need to call flush manually in each test, except when
testing code that creates warning without checking them.