bpo-43244: Remove ast.h, asdl.h, Python-ast.h headers (GH-24933)

These functions were undocumented and excluded from the limited C
API.

Most names defined by these header files were not prefixed by "Py"
and so could create names conflicts. For example, Python-ast.h
defined a "Yield" macro which was conflict with the "Yield" name used
by the Windows <winbase.h> header.

Use the Python ast module instead.

* Move Include/asdl.h to Include/internal/pycore_asdl.h.
* Move Include/Python-ast.h to Include/internal/pycore_ast.h.
* Remove ast.h header file.
* pycore_symtable.h no longer includes Python-ast.h.
diff --git a/configure b/configure
index 2ddca07..f9b2aff 100755
--- a/configure
+++ b/configure
@@ -2765,7 +2765,7 @@
     # If we're building out-of-tree, we need to make sure the following
     # resources get picked up before their $srcdir counterparts.
     #   Objects/ -> typeslots.inc
-    #   Include/ -> Python-ast.h
+    #   Include/ -> Python.h
     #   Python/  -> importlib.h
     # (A side effect of this is that these resources will automatically be
     #  regenerated when building out-of-tree, regardless of whether or not