Fix Qmatrix/TupleOfTuple var access error

[dakk]

This raises Not a tuple in ast2ast visit subscript with not constant _sval: Subscript (value=Name(id='a', ctx=Load()), slice=Name(id='i', ctx=Load()), ctx=Load())

@qlassf
def test(a: Qmatrix[Qint[2], 2, 2]) -> Qint[2]:
   i = 1
   j = i + 1
   return a[i][i]