gh-104533: Fix @ctypes.util.struct with endian (#154038)

Change `_fields_` type to a list of tuples, instead of a list of lists.

Modify test.test_ctypes.test_aligned_structures to test also
`@ctypes.util.struct` decorator.

Author: vstinner

Lib/ctypes/util.py

@@ -532,7 +532,8 @@ def _process_struct(decorated_class, /, *, align, layout, endian, pack):
         else:
             field.append(hint)
 
-        fields.append(field)
+        # _fields_ is a list of tuples
+        fields.append(tuple(field))
 
     if endian == 'big':
         endian_class = BigEndianStructure