Cannot use interface methods when returned through function returning `[]Interface` type

Reproduction steps:
1. Run commands:
```
mkdir dum
cd dum
go mod init dummy.com/dum
mkdir hello
```
2. Create file named `hello/hello.go` with following contents:
```go
package hello

type Example interface {
Sample() int
}

type ExampleImpl struct {
sample int
}

func (example *ExampleImpl) Sample() int {
return example.sample
}

func Hello() []Example {
return []Example{&ExampleImpl{sample: 123}, &ExampleImpl{sample: 321}}
}
```
3. Build with `gopy`:
```
gopy build -output=out dummy.com/dum/hello
```
4. Run the following Python code:
```pycon
>>> from out import hello
>>> output = hello.Hello()
>>> output[0].Sample()
panic: interface conversion: *hello.Example is not hello.Example: missing method Sample

goroutine 17 [running, locked to thread]:
main.hello_Example_Sample(0x2)
/tmp/dum/out/hello.go:1230 +0xf9
zsh: IOT instruction (core dumped) python
```

使用报告中的命令复现该问题，然后检查生成的 out/hello.go 中 hello_Example_Sample 附近的内容以及 hello.Hello 的 wrapper。验证 hello.Hello 返回的接口可以在两个元素上调用 Sample 而不会发生 panic，并返回 123 和 321。

由索引模型根据 Issue 内容生成。