gh-101819: Explicitly disallow pickle protocols 0 and 1 in _io

[erlend-aasland]

This is part of the preparations for isolating the _io module.

• Issue: Isolate the _io extension module #101819

[erlend-aasland]

See #84258 (comment) and #85224 (comment), for more info

[vstinner]

See #84258 (comment) and #85224 (comment), for more info

I'm not confortable to have to dig deep into the bug tracker to get reliable information about object serialization. Can it be written in the Python documentation instead?

Since these comments, I recall that @serhiy-storchaka made generic changes on base object type so it's no longer needed, no?

object.__reduce__() has now a default implementation which returns the string "NotImplemented". Is this change still needed?

[erlend-aasland]

object.__reduce__() has now a default implementation which returns the string "NotImplemented". Is this change still needed?

Hm, perhaps we only need to change the _io tests.

[erlend-aasland]

I believe you are right, @vstinner; there is no need for this.