gh-155558: Update bundled Expat to 2.8.3

[StanFromIreland]

• Issue: Upgrade bundled Expat to 2.8.3 (e.g. for the fix to CVE-2026-72522) #155558

[encukou]

Modules/expat/refresh.sh gives me the same result; changes in Expat look reasonable.

[miss-islington-app]

Thanks @StanFromIreland for the PR, and @encukou for merging it 🌮🎉.. I'm working now to backport this PR to: 3.10, 3.11, 3.12, 3.13, 3.14, 3.15.
🐍🍒⛏🤖

[bedevere-app]

GH-155612 is a backport of this pull request to the 3.15 branch.

[bedevere-app]

GH-155613 is a backport of this pull request to the 3.14 branch.

[bedevere-app]

GH-155614 is a backport of this pull request to the 3.13 branch.

[miss-islington-app]

Sorry, @StanFromIreland and @encukou, I could not cleanly backport this to 3.11 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker 3d6a50598ba7c89a6468f8e77afe2a39787c80e5 3.11

[bedevere-app]

GH-155615 is a backport of this pull request to the 3.12 branch.

[miss-islington-app]

Sorry, @StanFromIreland and @encukou, I could not cleanly backport this to 3.10 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker 3d6a50598ba7c89a6468f8e77afe2a39787c80e5 3.10

[StanFromIreland]

Modules/expat/refresh.sh gives me the same result

JFYI @encukou, we have a CI check for that https://github.com/python/cpython/actions/runs/31501678160 :-)

[bedevere-app]

GH-155616 is a backport of this pull request to the 3.11 branch.

[bedevere-app]

GH-155618 is a backport of this pull request to the 3.10 branch.

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 FreeBSD14 3.12 (tier-3) has failed when building commit c546712.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/1233/builds/1413) and take a look at the build logs.
4. Check if the failure is related to this commit (c546712) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/1233/builds/1413

Failed tests:

• test_httpservers

Failed subtests:

• test_large_content_length_truncated - test.test_httpservers.CGIHTTPServerTestCase.test_large_content_length_truncated

Summary of the results of the build (if available):

==

Click to see traceback logs

Traceback (most recent call last):
  File "/home/buildbot/buildarea/3.12.opsec-fbsd14/build/Lib/test/test_httpservers.py", line 914, in test_large_content_length_truncated
    res = self.request('/cgi-bin/file1.py', 'POST', b'x', headers)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/buildbot/buildarea/3.12.opsec-fbsd14/build/Lib/test/test_httpservers.py", line 87, in request
    return self.connection.getresponse()
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/buildbot/buildarea/3.12.opsec-fbsd14/build/Lib/http/client.py", line 1478, in getresponse
    response.begin()
  File "/home/buildbot/buildarea/3.12.opsec-fbsd14/build/Lib/http/client.py", line 343, in begin
    version, status, reason = self._read_status()
                              ^^^^^^^^^^^^^^^^^^^
  File "/home/buildbot/buildarea/3.12.opsec-fbsd14/build/Lib/http/client.py", line 312, in _read_status
    raise RemoteDisconnected("Remote end closed connection without"
http.client.RemoteDisconnected: Remote end closed connection without response


Traceback (most recent call last):
  File "/home/buildbot/buildarea/3.12.opsec-fbsd14/build/Lib/test/test_httpservers.py", line 914, in test_large_content_length_truncated
    res = self.request('/cgi-bin/file1.py', 'POST', b'x', headers)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/buildbot/buildarea/3.12.opsec-fbsd14/build/Lib/test/test_httpservers.py", line 87, in request
    return self.connection.getresponse()
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/buildbot/buildarea/3.12.opsec-fbsd14/build/Lib/http/client.py", line 1478, in getresponse
    response.begin()
  File "/home/buildbot/buildarea/3.12.opsec-fbsd14/build/Lib/http/client.py", line 343, in begin
    version, status, reason = self._read_status()
                              ^^^^^^^^^^^^^^^^^^^
  File "/home/buildbot/buildarea/3.12.opsec-fbsd14/build/Lib/http/client.py", line 304, in _read_status
    line = str(self.fp.readline(_MAXLINE + 1), "iso-8859-1")
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/buildbot/buildarea/3.12.opsec-fbsd14/build/Lib/socket.py", line 720, in readinto
    return self._sock.recv_into(b)
           ^^^^^^^^^^^^^^^^^^^^^^^
ConnectionResetError: [Errno 54] Connection reset by peer