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:

• etree: Defining the default namespace prefix ("") with register_namespace has issues #118416 (bug)
• ElementTree incorrectly refuses to write attributes without namespaces when default_namespace is used #61290 (bug)
• ElementTree.tostring(..., default_namespace=...) incorrectly strips namespace from attributes #113581 (fix for 61290 also fixes this)
• ET: add custom namespaces to serialization methods #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-120771: Refactor xml.etree.ElementTree._namespaces #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): danifus#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.

[joooeey]

This may be related: #132637