Message 83546 - Python tracker

I'm thinking of allowing you to mix keywords and auto-numbering, but not
manual numbering and auto-numbering. This would be so we could do:

>>> '{:{fmt}} {:{fmt}}'.format(3.1415, 2.71828, fmt='1.4f')
'pi=3.1415 e=2.7183'

Unfortunately the ':' is required, because if it's not there you have 2
braces next to each other, which is the escape sequence for a single brace.