Issue 5237, Allow auto-numbered replacement fields in str.format() strings.

Issue 5237, Allow auto-numbered replacement fields in str.format() strings.

For simple uses for str.format(), this makes the typing easier. Hopfully this
will help in the adoption of str.format().

For example:
'The {} is {}'.format('sky', 'blue')

You can mix and matcth auto-numbering and named replacement fields:
'The {} is {color}'.format('sky', color='blue')

But you can't mix and match auto-numbering and specified numbering:
'The {0} is {}'.format('sky', 'blue')
ValueError: cannot switch from manual field specification to automatic field numbering

Will port to 3.1.