@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.