gh-132775: Add _PyCode_ReturnsOnlyNone() (gh-132981)

The function indicates whether or not the function has a return statement.

This is used by a later change related treating some functions like scripts.

Python/flowgraph.c

@@ -295,7 +295,7 @@ dump_instr(cfg_instr *i)
 static inline int
 basicblock_returns(const basicblock *b) {
     cfg_instr *last = basicblock_last_instr(b);
-    return last && last->i_opcode == RETURN_VALUE;
+    return last && IS_RETURN_OPCODE(last->i_opcode);
 }
 
 static void