`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

Bắt đầu tại Lib/xml/dom/domreg.py, ở các dòng 73-76, và xem xét vòng lặp fallback được mô tả trong issue. Xác minh rằng chỉ các exception thiếu triển khai đã nêu mới bị bỏ qua, trong khi các lỗi khác vẫn hiển thị; chạy các bài kiểm thử XML DOM liên quan hoặc reproducer được cung cấp để xác nhận hành vi.