py/qstr: Use const consistently to avoid a cast.

Originally at adafruit#4707

Signed-off-by: Artyom Skrobov <tyomitch@gmail.com>

py/qstr.h

@@ -64,7 +64,7 @@ typedef uint16_t qstr_len_t;
 #endif
 
 typedef struct _qstr_pool_t {
-    struct _qstr_pool_t *prev;
+    const struct _qstr_pool_t *prev;
     size_t total_prev_len;
     size_t alloc;
     size_t len;