> I don't understand what the problem is. _pyio.open is a function not a static method.

The problem is that _pyio.open doesn't behave exactly as io.open when it's used to define a method:
---
#from io import open
from _pyio import open

class MyClass:
   my_open = open

MyClass().my_open("document.txt", "w")
---

This code currently fails with a TypeError, whereas it works with io.open.

The problem is that I failed to find a way to create a function in Python which behaves exactly as built-in functions like len() or io.open().