[python] AttributeError for a class without constructor

```haxe
class Main {
static function main() {
var c = new Bar();
trace(c.foo());
}
}

class Bar extends Foo {
public function new() {}
}

class Foo {
var v:Int;
public function foo() {
return v == null;
}
}
```

```
Traceback (most recent call last):
File "bin/python/tests.py", line 43, in
Main.main()
File "bin/python/tests.py", line 11, in main
print(str(Bar().foo()))
File "bin/python/tests.py", line 18, in foo
return (self.v is None)
AttributeError: v
```