Issue 28974: Make default __format__ be equivalent to __str__

Originally PEP 3101 defined the default __format__ implementation, object.__format__ as

     def __format__(self, format_spec):
         return format(str(self), format_spec)

After few changes (issue7994, issue28385) it now looks as

     def __format__(self, format_spec):
         assert format_spec == ''
         return format(str(self), '')

Proposed patch makes it yet simpler:

     def __format__(self, format_spec):
         assert format_spec == ''
         return str(self)

This is equivalent to the previous form except obscure case when str() returns not exact str, but strict subclass with overridden __format__.

The benefit of this change is simpler semantical model of the default implementation.

See the start of the discussion in issue28385 and the discussion on Python-Dev: https://mail.python.org/pipermail/python-dev/2016-October/146765.html.

What are your thoughts about this Eric?

I'm okay with this change.

New changeset 7e19dbc92ec06a987eaae72f7cdfd32006aa4960 by Serhiy Storchaka in branch 'master':
bpo-28974: `object.__format__(x, '')` is now equivalent to `str(x)` (#506)
https://github.com/python/cpython/commit/7e19dbc92ec06a987eaae72f7cdfd32006aa4960