Doc: C API: Demote sections to subsections for consistency (pythonGH-…

…126535)

The entire file should be a single section; the headings below the
first heading should be subsections.
(cherry picked from commit e3510bd)

Co-authored-by: Richard Hansen <rhansen@rhansen.org>

Doc/c-api/typeobj.rst

@@ -2193,7 +2193,7 @@ This is done by filling a :c:type:`PyType_Spec` structure and calling
 .. _number-structs:
 
 Number Object Structures
-========================
+------------------------
 
 .. sectionauthor:: Amaury Forgeot d'Arc
 
@@ -2307,7 +2307,7 @@ Number Object Structures
 .. _mapping-structs:
 
 Mapping Object Structures
-=========================
+-------------------------
 
 .. sectionauthor:: Amaury Forgeot d'Arc
 
@@ -2344,7 +2344,7 @@ Mapping Object Structures
 .. _sequence-structs:
 
 Sequence Object Structures
-==========================
+--------------------------
 
 .. sectionauthor:: Amaury Forgeot d'Arc
 
@@ -2424,7 +2424,7 @@ Sequence Object Structures
 .. _buffer-structs:
 
 Buffer Object Structures
-========================
+------------------------
 
 .. sectionauthor:: Greg J. Stein <greg@lyra.org>
 .. sectionauthor:: Benjamin Peterson
@@ -2519,7 +2519,7 @@ Buffer Object Structures
 
 
 Async Object Structures
-=======================
+-----------------------
 
 .. sectionauthor:: Yury Selivanov <yselivanov@sprymix.com>
 
@@ -2587,7 +2587,7 @@ Async Object Structures
 .. _slot-typedefs:
 
 Slot Type typedefs
-==================
+------------------
 
 .. c:type:: PyObject *(*allocfunc)(PyTypeObject *cls, Py_ssize_t nitems)
 
@@ -2696,7 +2696,7 @@ Slot Type typedefs
 .. _typedef-examples:
 
 Examples
-========
+--------
 
 The following are simple examples of Python type definitions.  They
 include common usage you may encounter.  Some demonstrate tricky corner