bpo-43680: _pyio.open() becomes a static method

[vstinner]

The Python _pyio.open() function becomes a static method to behave as
io.open() built-in function: don't become a bound method when stored
as a class variable. It becomes possible since static methods are now
callable in Python 3.10. Moreover, _pyio.OpenWrapper becomes a simple
alias to _pyio.open.

init_set_builtins_open() now sets builtins.open to io.open, rather
than setting it to io.OpenWrapper, since OpenWrapper is now an alias
to open in the io and _pyio modules.

https://bugs.python.org/issue43680

[vstinner]

@methane: Would you mind to review this change?

Thanks to https://bugs.python.org/issue43682, it becomes possible to put @staticmethod on _pyio.open. I propose to start with this change. Then I will propose a change to deprecate io.OpenWrapper and _pyio.OpenWrapper.

[methane]

LGTM except one nitpick.

[methane]

Can we remove DocDescriptor too?

It is not defined in C _io module. So I think it is safe to remove.

[vstinner]

Can we remove DocDescriptor too?
It is not defined in C _io module. So I think it is safe to remove.

Sure, it was not documented and doesn't exist in the io module. I already removed it: 3bc694d