Methods with two outputs are expected to return an error, methods aren't visible from python

Hi, I'm trying to build this project, https://github.com/holiman/uint256, but I've run into multiple issues:

Suppose we have a function with multiple outputs: `func (z *Int) AddOverflow(x, y *Int) (*Int, bool)`, bind expects second argument to be an `error` and outputs the following `gopy: second result value must be of type error`.

To reproduce, minimized testcase:
```go
package uint256

import (
"math/bits"
)

type Int [4]uint64

func NewInt(val uint64) *Int {
z := &Int{}
z.SetUint64(val)
return z
}

func (z *Int) SetUint64(x uint64) *Int {
z[3], z[2], z[1], z[0] = 0, 0, 0, x
return z
}

func (z *Int) AddOverflow(x, y *Int) (*Int, bool) {
var carry uint64
z[0], carry = bits.Add64(x[0], y[0], 0)
z[1], carry = bits.Add64(x[1], y[1], carry)
z[2], carry = bits.Add64(x[2], y[2], carry)
z[3], carry = bits.Add64(x[3], y[3], carry)
return z, carry != 0
}
```

The second issue I'm having is that after compiling the project, public methods of type `Int` aren't exposed to Python, e.g. the following code won't run:
```python
from uint256_py import uint256

x, y, z = uint256.NewInt(123), uint256.NewInt(456), uint256.NewInt(0)
z.Add(x, y) # doesn't work, method isn't visible
```

I suspect this has something to do with `Int` being represented as an array `[4]uint64`.
Versions I'm using: Go 1.21.3 & Python 3.11.5

It seemed related to #315, so I've run this in docker with Go 1.19.5 & Python3.10.12 but the issue persisted.

Thank you.

Start with the minimized Go package and the generated Python import and method call shown in the issue. Inspect gopy's handling of multiple return values and methods on array-backed types; done means the testcase binds the (*Int, bool) result and exposes Int methods to Python.

Written by the indexing model from the issue text.