gh-36109: Prepare for updating Cython to 3.0.0
    
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Here are the changes needed to get Sage compiled with Cython 3. This PR
contains changes that do not break compatibility with old Cython, so
they can be merged without actually upgrading Cython.

Part of #29863.

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URL: #36109
Reported by: Yutao Yuan
Reviewer(s): Gonzalo Tornaría, Yutao Yuan

Author: Release Manager

src/doc/en/thematic_tutorials/coercion_and_categories.rst

@@ -106,7 +106,6 @@ This base class provides a lot more methods than a general parent::
 
     sage: [p for p in dir(Field) if p not in dir(Parent)]
     ['__fraction_field',
-     '__ideal_monoid',
      '__iter__',
      '__len__',
      '__rxor__',
@@ -119,6 +118,7 @@ This base class provides a lot more methods than a general parent::
      '_default_category',
      '_gens',
      '_ideal_class_',
+     '_ideal_monoid',
      '_latex_names',
      '_list',
      '_one_element',

src/doc/en/thematic_tutorials/tutorial-objects-and-classes.rst

@@ -298,7 +298,7 @@ http://docs.python.org/library/ for a complete list. ::
         sage: el
         bla
         sage: el.__dict__
-        {'__custom_name': 'bla', 'value': 42}
+        {'_SageObject__custom_name': 'bla', 'value': 42}
 
     Lots of Sage objects are not Python objects but compiled Cython
     objects. Python sees them as builtin objects and you do not have