Convert all dunder functions to use the normal function signature

[thepabloaguilar]

When we define a class like this:

def func():
    ...

class MyClass:
    my_func = lambda self, arg: arg

# or
class MyAnotherClass:
    my_func = func

The coverage plugin will consider the my_func as coveraged without any tests.
This behavior is expected because for it the my_func is an attribute!

I think it will be great use the normal signature, this will enforce we to make dummy tests to ensures the _ will work correctly.