bpo-43244: Add pycore_ast.h header file (GH-24908)

Move _PyAST_GetDocString() and _PyAST_ExprAsUnicode() functions the
internal C API: from Include/ast.h to a new
Include/internal/pycore_ast.h header file. Don't export these
functions anymore: replace PyAPI_FUNC() with extern.

Remove also unused includes.
diff --git a/Python/ast_opt.c b/Python/ast_opt.c
index 6ad00dc..c76b428 100644
--- a/Python/ast_opt.c
+++ b/Python/ast_opt.c
@@ -1,7 +1,6 @@
 /* AST Optimizer */
 #include "Python.h"
-#include "Python-ast.h"
-#include "ast.h"
+#include "pycore_ast.h"           // _PyAST_GetDocString()
 
 
 static int