Support fractions in string formatting

PR #100161 is now merged. Thanks to all for comments, and especially to @ericvsmith for review.

Python 3.12.0a4+ (heads/main:3e09f3152e, Jan 22 2023, 19:39:49) [Clang 14.0.0 (clang-1400.0.29.202)] on darwin
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>>> from fractions import Fraction
>>> f'{Fraction(2635, 1000):.2f}'
'2.64'
>>> f'{Fraction(355, 113):.25f}'
'3.1415929203539823008849558'
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