unbound method __init__() error with python2 but not with python3

This is related to my earlier question: https://stackoverflow.com/questions/76324839/unbound-method-init-error-in-unit-tests-but-not-in-regular-executions?noredirect=1#comment134593579_76324839

I made a minimal reproducible example below:

class Base:
  def __init__(self):
    print("base constructor")

class derived(Base):
  def __init__(self):
    print("derived constructor")

class Secondary(Base):
  def __init__(self):
    derived.__init__(self)

s = Secondary()

If you run this with python3.7 (or any python3 version I think), it works fine, but with python2.7, you get the error

$ python2.7 test.py
Traceback (most recent call last):
  File "test.py", line 13, in <module>
    s = Secondary()
  File "test.py", line 11, in __init__
    derived.__init__(self)
TypeError: unbound method __init__() must be called with derived instance as first argument (got Secondary instance instead)

Why does this work with python3 but not python2? Is there a way to make this work with python2?

Also tried using super (had to make Base inherit object to use it):

class Base(object):
  def __init__(self):
    print("base constructor")

class derived(Base):
  def __init__(self):
    print("derived constructor")

class Secondary(Base):
  def __init__(self):
    super(derived, self).__init__()

s = Secondary()

There's still an issue with python2. The error changes (similar meaning I think):

$ python2.7 test.py
Traceback (most recent call last):
  File "test.py", line 13, in <module>
    s = Secondary()
  File "test.py", line 11, in __init__
    super(derived, self).__init__()
TypeError: super(type, obj): obj must be an instance or subtype of type

In Python 2, accessing a function-valued class attribute always produces an instance of instancemethod, whether you access it via the class or an instance of the class.

>>> class Foo(object):
...   def __init__(self):
...     pass
...
>>> type(Foo.__init__)
<type 'instancemethod'>
>>> type(Foo().__init__)
<type 'instancemethod'>

The only difference is whether the instancemethod has a reference to an object that can be passed to the function when you call the instancemethod.

>>> f = Foo()
>>> Foo.__init__()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unbound method __init__() must be called with Foo instance as first argument (got nothing instead)
>>> Foo.__init__(f)  # OK
>>> f.__init__()  # OK

In Python 3, accessing a function-valued class attribute on a class produces the raw function; only when you access it via an instance of the class do you get an instance of method.

>>> class Foo:
...   def __init__(self):
...     pass
...
>>> type(Foo.__init__)
<class 'function'>
>>> type(Foo().__init__)
<class 'method'>