gh-67790: Add integer-style formatting for fractions.Fraction

[mdickinson]

This is a proof-of-concept PR that extends python#100161 to add support for the 'd' presentation type in fractions.Fraction.__format__.

Detailed changes

• Add 'd' presentation type, supporting alignment, fill, sign and minimum width in the obvious-ish way (minimum width applies to the whole formatted number).
• Add support for # alternate formatting, which always includes a slash and denominator (by default, if the denominator is 1 then the slash and denominator are left out).
• Include support for thousands separators, with the separator applied to both numerator and denominator separately.
• Do not support zero padding, since it's not clear exactly how that would work or what the use-cases are.
• Make 'd' the default presentation type: a missing presentation type behaves identically to 'd'.

Feature complete and tested, but not yet ready to merge - it still needs doc updates.

To do

• Update what's new entry
• Update docs
• Update news entry (or create a new one if there's been a release since merge of gh-67790: Support float-style formatting for Fraction instances python/cpython#100161)

[mdickinson]

(Not very) wild idea: we could add a new alignment type / that aligns fractions at the slash (to the extent possible within the given minimum width).

[mdickinson]

Re-opened as python#111320

[mdickinson]

we could add a new alignment type / that aligns fractions at the slash

Not doing this - no-one has asked for it, and there are some details that are unclear (like where the sign should go).