TypeConversion: Fix pointer qualifier compiler warning

This fixes the following compiler warning:
  warning: cast discards ‘const’ qualifier from pointer target type [-Wcast-qual]

Cython/Utility/TypeConversion.c

@@ -730,14 +730,14 @@ static CYTHON_INLINE PyObject* {{TO_PY_FUNCTION}}({{TYPE}} value, Py_ssize_t wid
             digit_pos = abs((int)(remaining % (8*8)));
             remaining = ({{TYPE}}) (remaining / (8*8));
             dpos -= 2;
-            *(uint16_t*)dpos = ((uint16_t*)DIGIT_PAIRS_8)[digit_pos]; /* copy 2 digits at a time */
+            *(uint16_t*)dpos = ((const uint16_t*)DIGIT_PAIRS_8)[digit_pos]; /* copy 2 digits at a time */
             last_one_off = (digit_pos < 8);
             break;
         case 'd':
             digit_pos = abs((int)(remaining % (10*10)));
             remaining = ({{TYPE}}) (remaining / (10*10));
             dpos -= 2;
-            *(uint16_t*)dpos = ((uint16_t*)DIGIT_PAIRS_10)[digit_pos]; /* copy 2 digits at a time */
+            *(uint16_t*)dpos = ((const uint16_t*)DIGIT_PAIRS_10)[digit_pos]; /* copy 2 digits at a time */
             last_one_off = (digit_pos < 10);
             break;
         case 'x':