`xml.dom.domreg.getDOMImplementation`: fallback discovery masks genuine bugs, not just "not installed"

Bug report

Bug description:

• Location: Lib/xml/dom/domreg.py:73-76

https://github.com/python/cpython/blob/cbe3a7f06a38c7fdda2d53183294499e533a3a0c/Lib/xml/dom/domreg.py#L72-L78

• Problem: When called with no explicit implementation name, tries each well-known implementation (minidom, 4DOM) in turn under except Exception: continue (comment: "typically ImportError, or AttributeError"). A genuine bug in a candidate's getDOMImplementation() factory (e.g. a real AttributeError from a typo) is caught identically to "this implementation isn't available" and silently skipped.
• Reproducer (verified): registered a fake well-known implementation whose getDOMImplementation() deliberately raises AttributeError to simulate a real bug (not a missing-module ImportError); the fallback loop silently moved on to minidom with no trace of the failure.
• Suggested fix: Catch (ImportError, AttributeError) specifically, matching what the comment already says is "typical."
• Difficulty: Trivial. Note: low real-world impact since well_known_implementations is a tiny, fixed, mostly-dead-code dict — good for a first-time contributor precisely because it's low-stakes.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-155948

Lib/xml/dom/domreg.py の 73-76 行目から始め、issue で説明されているフォールバックループを確認してください。指定された実装欠如の例外だけがスキップされ、その他の失敗は引き続き可視のままであることを検証してください。動作を確認するため、関連する XML DOM テストまたは提供された reproducer を実行してください。