Use proper markup for the C "constants" #106919

Many of the C "constants" (actually macros without parameters) are declared in the documentation using the data directive and referred using :data: and :const: roles. It is incorrect, because these directive and roles are defined in the Python domain and purposed to use with Python module level variables. I think that it is better to document them as they are, the C macros.

One of the differences is that the HTML anchors contains the c. prefix, e.g. c.METH_VARARGS. There may also be the difference in visual representation.

Other issues in the current markup:

• In the index they are marked as "(built-in variable)".
• There are no index entries for METH_KEYWORDS and METH_METHOD and they cannot be referred.
• There are two definitions of Py_TPFLAGS_HAVE_GC. They does not conflict only because they are in different namespaces.

Linked PRs

• gh-106919: Use role :c:macro: for referencing the C "constants" #106920
• [3.12] gh-106919: Use role :c:macro: for referencing the C "constants" (GH-106920) #106951
• [3.11] gh-106919: Use role :c:macro: for referencing the C "constants" (GH-106920) #106952