bpo-67790: Add float-style formatting for Fraction objects

[mdickinson]

This work-in-progress / proof-of-concept PR adds float-style formatting for fractions.Fraction objects.

So far this includes support for the 'e', 'f', 'g', and '%' presentation types. The 'n' presentation type is not yet supported.

Python 3.12.0a2+ (heads/fraction-format:8cf8d31ce7, Dec  3 2022, 17:24:40) [Clang 14.0.0 (clang-1400.0.29.202)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from fractions import Fraction as F
>>> f = F(22, 7)
>>> format(f, 'f')
'3.142857'
>>> format(f, 'e')
'3.142857e+00'
>>> f'{f:f}'  
'3.142857'
>>> format(f, '.50f')
'3.14285714285714285714285714285714285714285714285714'
>>> format(F('123456789.12345678'), '_.6f')
'123_456_789.123457'
>>> format(f, '020,f')
'0,000,000,003.142857'
>>> format(f, 'X^20f')
'XXXXXX3.142857XXXXXX'
>>> format(F(2**10000), 'e')
'1.995063e+3010'

It supports all reasonable bells and whistles of the formatting mini-language.

[mdickinson]

Updated to add '%' formatting, and to add the 'F' format specifier as an alias of 'f'.

[mdickinson]

PR recreated against upstream: python#100161. Closing here.