bpo-2897: Make PyMemberDef part of stable ABI; deprecate structmember.h

[MatzeB]

Move PyMemberDef and related flags from structmember.h to descrobject.h.
Rename READONLY to PY_READONLY and READ_RESTRICTED to
PY_READ_RESTRICTED. This makes the structure part of the stable ABI.

Move PyMember_GetOne and PyMemberSetOne as well but do not make them
part of the stable ABI yet.

Document that structmember.h is deprecated, change examples and
documentation and add definitions for READONLY, READ_RESTRICTED,
PY_WRITE_RESTRICTED and RESTRICTED to it for backwards compatibility.

Add check for stddef.h to configure.ac because we need to include it for
offsetof and Python.h does not pass the HAVE_STDDEF_H check without it.

https://bugs.python.org/issue2897

[encukou]

Hello. Sorry for the delay -- I just found this PR (I noticed the bug and wanted to file an issue myself).
Are you still interested in the PR, after 2 years?
If not, I'd be happy to take over.

• Changing all the uses of READONLY to PY_READONLY (and removing the #include) isn't really necessary. The PR would be much easier to review without that, and there'd be fewer conflicts.
• Everything included from Python.h should have the Py_ prefix, so T_* macros should be renamed to PY_T_*. The old header should continue to provide the old names.
  • The new constants should be documented and added to Misc/stable_abi.toml, but let's do that in a separate PR.
• We should ensure we don't reuse PY_WRITE_RESTRICTED's value for something else in the future. So, in descrobject.h, I would #define _PY_WRITE_RESTRICTED 4 with an appropriate comment. (Note the leading underscore.)
• The configure*, Python.h and HAVE_STDDEF_H stuff shouldn't change here. structmember.h should include stddef.h for backward compatibility, but the other stuff can be left to other PRs/issues, if necessary.
• This needs an entry in What's New.

[MatzeB]

Hey, I am currently not working with python so I don't know when I would find the time to push this further. If you or someone else wants to take over, by all means!

[encukou]

Taking over in #99014