Please upgrade bundled Expat to 2.7.3 (e.g. for two key bugfixes)

[hartwork]

Bug report

Bug description:

Hello! 👋

(Once the upgrade to Expat 2.7.2 is complete at #138998) please upgrade bundled Expat to 2.7.3 (e.g. for two key bugfixes).

• GitHub release: https://github.com/libexpat/libexpat/releases/tag/R_2_7_3
• Change log: https://github.com/libexpat/libexpat/blob/R_2_7_3/expat/Changes

The CPython issue for previous 2.7.2 was #138998 and the related merged main pull request was #138999, in case you want to have a look. (The Dockerfile from comment #123689 (review) could be of help with raising confidence in a bump pull request when going forward.)

Thanks in advance!

CC @gpshead @StanFromIreland

CPython versions tested on:

3.9, 3.10, 3.11, 3.12, 3.13, 3.14, 3.15, CPython main branch

Operating systems tested on:

Linux, macOS, Windows, Other

Linked PRs

• gh-139312: Update bundled libexpat to 2.7.3 #139319
• [3.14] gh-139312: Update bundled libexpat to 2.7.3 (GH-139319) #139376
• [3.12] gh-139312: Update bundled libexpat to 2.7.3 (GH-139319) #139662
• [3.13] gh-139312: Update bundled libexpat to 2.7.3 (GH-139319) #139377
• [3.11] gh-139312: Update bundled libexpat to 2.7.3 (GH-139319) #139378
• [3.10] gh-139312: Update bundled libexpat to 2.7.3 (GH-139319) #139379

[picnixz]

I'm not adding the 3.9 tag because I don't think we'll backport 2.7.2 to 3.9 considering it's EOL in October.