bpo-42747: Remove Py_TPFLAGS_HAVE_AM_SEND and make Py_TPFLAGS_HAVE_VERSION_TAG no-op

[encukou]

• Remove code that checks Py_TPFLAGS_HAVE_VERSION_TAG

  The field is always present in the type struct, as explained
  in the added comment.

• Remove Py_TPFLAGS_HAVE_AM_SEND

  The flag is not needed, and since it was added in 3.10 it can be removed now.

https://bugs.python.org/issue42747

Automerge-Triggered-By: GH:encukou

[pablogsal]

How this flag was added and when it was turned into a no-op?

[encukou]

Py_TPFLAGS_HAVE_* flags have been historically added whenever a new field was added to the type struct, to make it possible to use extensions compiled for earlier Python versions (where the fields aren't present).
But extensions need to be recompiled for each feature version (as long I can remember, and definitely in Python 3.0+), or use the stable ABI (where types use the interpreter's memory layout).

This PR turns both into no-ops, as they should be.
One of the removed code paths probably has an issue, see bpo-44588 -- but I didn't investigate that deeper yet.

[shihai1991]

Py_TPFLAGS_HAVE_AM_SEND flag (which was added in 3.10) is removed.

there aren't many users use it?

[encukou]

If there any users of it, they are using the 3.10 betas. Requiring that they remove the flag is fair game.

[miss-islington]

Thanks @encukou for the PR 🌮🎉.. I'm working now to backport this PR to: 3.10.
🐍🍒⛏🤖

[bedevere-bot]

GH-27306 is a backport of this pull request to the 3.10 branch.

[vstinner]

Thanks for this fix @encukou. The rationale for Py_TPFLAGS_HAVE_AM_SEND was unclear. Now it's well defined, it's useless :-D