lxml.etree._Attrib doesn't inherit Mapping

[Pekka Klärck]

We have code that operates with various collections and when working with dictionaries it checks is the object a dictionary with `isinstance(obj, Mapping)`. That works fine in general, including with the standard ElementTree, but fails with `lxml.etree._Attrib`. To reproduce:

>>> from collections.abc import Mapping
>>> from xml.etree.ElementTree import XML
>>> isinstance(XML('<x/>').attrib, Mapping)
True
>>> from lxml.etree import XML
>>> isinstance(XML('<x/>').attrib, Mapping)
False

Required version info:

Python : sys.version_info(major=3, minor=8, micro=10, releaselevel='final', serial=0)
lxml.etree : (4, 9, 1, 0)
libxml used : (2, 9, 14)
libxml compiled : (2, 9, 14)
libxslt used : (1, 1, 35)
libxslt compiled : (1, 1, 35)

[scoder (scoder)]

Right, it should register as MutableMapping.

https://github.com/lxml/lxml/commit/ff82753bd09408ecfd6cacf7da67257495dc1335