Patched PropertyDocumenter fails on properties created using @functools.cached_property

[antonymayi]

Sphinx analyses properties created with the @functools.cached_property as a normal property using the PropertyDocumenter which sphinx-immaterial patches with additional _get_property_return_type function. That assumes the property object has a .fget attribute/method which, however, is not the case for cached_property objects, hence failing with AttributeError:

  File "/lib/python3.9/site-packages/sphinx/ext/autodoc/__init__.py", line 1792, in document_members
    super().document_members(all_members)
  File "/lib/python3.9/site-packages/sphinx/ext/autodoc/__init__.py", line 857, in document_members
    documenter.generate(
  File "/lib/python3.9/site-packages/sphinx/ext/autodoc/__init__.py", line 910, in generate
    if not self.import_object():
  File "/lib/python3.9/site-packages/sphinx_immaterial/autodoc_property_type.py", line 41, in import_object
    self.retann = _get_property_return_type(self.object)
  File "/lib/python3.9/site-packages/sphinx_immaterial/autodoc_property_type.py", line 15, in _get_property_return_type
    if obj.fget is None:
AttributeError: 'cached_property' object has no attribute 'fget'