`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

从 Lib/xml/dom/domreg.py 的第 73-76 行开始，检查 issue 中描述的回退循环。验证仅跳过所述的缺少实现异常，而其他失败仍保持可见；运行相关的 XML DOM 测试或提供的 reproducer，以确认该行为。