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/future.c b/Python/future.c
index 4b73eb6..37e7a22 100644
--- a/Python/future.c
+++ b/Python/future.c
@@ -1,9 +1,5 @@
 #include "Python.h"
-#include "Python-ast.h"
-#include "token.h"
-#include "code.h"
-#include "symtable.h"
-#include "ast.h"
+#include "pycore_ast.h"           // _PyAST_GetDocString()
 
 #define UNDEFINED_FUTURE_FEATURE "future feature %.100s is not defined"
 #define ERR_LATE_FUTURE \