@pydef not overloading method correctly

The following code does not work.

```julia
@pydef type SDTExpression <: fenics.Expression
eval(self, value, x) = begin
value[1] = 1.0
return
end
end
```

The error it gives is as follows.
```
TypeError("expected an overload 'eval' or 'eval_cell' method",)
```

This makes no sense because I've defined one.

Start by reproducing the Julia snippet using the @pydef entry point and the fenics.Expression subclass shown in the issue. Trace how @pydef discovers eval and eval_cell overloads, then verify that the defined eval method is accepted without the reported TypeError.

索引モデルが issue の本文から書いたものです。