Refactor `xml.etree.ElementTree._namespaces`

[danifus]

Feature or enhancement

Proposal:

Refactor of xml.etree.ElementTree._namespaces in preparation to simplify the logic to address the following gh issues, all of which require some changes to _namespaces:

• https://github.com/python/cpython/issues/118416 (bug)
• https://github.com/python/cpython/issues/61290 (bug)
• https://github.com/python/cpython/issues/113581 (fix for 61290 also fixes this)
• https://github.com/python/cpython/issues/57587 (new feature)

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Linked PRs

• gh-120772

[danifus]

As some more context about the benefits of the refactor, I've created another branch that includes the fixes for all the issues listed above based on this refactor (as a PR on my own fork): https://github.com/danifus/cpython/pull/3

If this approach looks good I'm happy to break this other branch down into a couple of separate PRs for each of the issues above and apply any suggested changes.