Add ExpatError and error to pyexpat

[fanninpm]

This PR is one of a series of PRs in preparation for getting https://rustpython.github.io/pages/regression-tests-results.html to work again by using the Python test suite to generate a JUnit-compatible XML file (and later converting said XML a JSON format the current website can accept).
In pursuit of that, I discovered that xml.parsers.expat.error needs to be available to the interpreter, and I suspect that it's used to let the Python-side XML machinery report a useful error.
Here, I tried to make pyexpat.error available; however, when I submitted this PR, I came up short of my goal, as I currently don't know a useful way of getting pyexpat.error to ailias pyexpat.ExpatError.
In CPython 3.14, the pyexpat module has a pyexpat_state struct that gets initialized with, among other things, a new xml.parsers.expat.ExpatError exception.
When the pyxpat module is initialized, the two objects error and ExpatError are aliased to state->Error.
If anyone knows how to replicate this behavior in RustPython on the Rust side, I would be glad to implement that.

Summary by CodeRabbit

• New Features
  • Added a new ExpatError exception type for improved error handling in XML parsing operations.

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[coderabbitai]

📝 Walkthrough

Walkthrough

A new PyExpatError exception type is introduced to the _pyexpat module and exported as ExpatError. The change includes expanding imports to include PyException from builtins and reorganizing public symbols in the module prelude.

Changes

Cohort / File(s)	Summary
New PyExpatError exception type crates/stdlib/src/pyexpat.rs	Added public PyExpatError struct deriving Debug, annotated with #[pyexception] and exported as ExpatError. Expanded imports to include PyException from builtins. Reorganized prelude exports to include the new exception type. Includes TODO for module state association.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Suggested reviewers

• youknowone

Poem

🐰 A new exception hops into view,
PyExpatError, shiny and new,
In the _pyexpat burrow it'll stay,
Catching parser woes, come what may! ✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The PR title 'Add ExpatError and error to pyexpat' accurately describes the main objective: introducing ExpatError exception and making error available in the pyexpat module.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing touches

• 📝 Generate docstrings

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[github-actions]

📦 Library Dependencies

The following Lib/ modules were modified. Here are their dependencies:

[ ] lib: cpython/Lib/xml
[ ] test: cpython/Lib/test/test_xml_etree.py (TODO: 56)
[ ] test: cpython/Lib/test/test_xml_etree_c.py
[ ] test: cpython/Lib/test/test_minidom.py
[ ] test: cpython/Lib/test/test_pulldom.py (TODO: 4)
[ ] test: cpython/Lib/test/test_pyexpat.py (TODO: 28)
[ ] test: cpython/Lib/test/test_sax.py

dependencies:

• xml (native: collections.abc, pyexpat, sys, urllib.parse, xml.dom, xml.dom.NodeFilter, xml.dom.minicompat, xml.dom.minidom, xml.dom.xmlbuilder, xml.etree.ElementTree, xml.parsers, xml.sax, xml.sax._exceptions, xml.sax.handler)
  • codecs (native: builtins, sys)
  • collections (native: _weakref, itertools, sys)
  • copy
  • io (native: _io, _thread, errno, sys)
  • os (native: os.path, sys)
  • weakref (native: _weakref, itertools, sys)
  • contextlib, re, warnings

dependent tests: (7 tests)

• xml: test_pulldom test_pyexpat test_regrtest test_xml_dom_minicompat test_xml_dom_xmlbuilder
  • plistlib: test_plistlib
  • xmlrpc.client: test_xmlrpc

Legend:

• [+] path exists in CPython
• [x] up-to-date, [ ] outdated