lxml adds duplicate namespace parameters

[kesara]

Since lxml 5.0.0, lxml adds duplicate namespace parameters.
This behaviour seems to happen only when DTD is defined and `attribute_defaults` is set to `True`.

output of test.py:
```
Python : sys.version_info(major=3, minor=10, micro=13, releaselevel='final', serial=0)
lxml.etree : (5, 1, 0, 0)
libxml used : (2, 12, 3)
libxml compiled : (2, 12, 3)
libxslt used : (1, 1, 39)
libxslt compiled : (1, 1, 39)
[('name', 'foo'), ('{http://www.w3.org/XML/1998/namespace}space', 'preserve'), ('{http://www.w3.org/XML/1998/namespace}space', 'preserve')]
```

`root[0].items()` has duplicate `'{http://www.w3.org/XML/1998/namespace}space` entries.

[scoder (scoder)]

This is most likely not due to a change in lxml but in libxml2. lxml 5.0 uses libxml2 2.12.3.

It might be related to this change:
https://gitlab.gnome.org/GNOME/libxml2/-/commit/8d42c447d616fb81c6f8f84a491ef50d882366f5