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.

Python/ast.c

@@ -3,8 +3,7 @@
  * of the given abstract syntax tree (potentially constructed manually).
  */
 #include "Python.h"
-#include "Python-ast.h"
-#include "ast.h"
+#include "pycore_ast.h"           // asdl_stmt_seq
 
 #include <assert.h>