It would be helpful to have the name provide a cue that a macro is being
used.

Agreed.

I assume that applies to Py_Refcnt and Py_Type as well?

I think it would be easier to merge from trunk to py3k before the change
and skip the revision in the next merge. The rename can be done with a
simple find | xargs sed -i. A merge might be more painful.

I tried a simple find | xargs sed replace and it worked well.

find -name '*.c' -or -name '*.h' -or -name '*.rst' | xargs sed -i
s/Py_Size\(/Py_SIZE\(/g

I've renamed the three macros and added the old names for b/w compatibility.

Why the compatibility?

FWIW, should PyUnicode_Check, PyTuple_Check etc. also change to
PyUnicode_CHECK, PyTuple_CHECK etc because they are macros?

I've fixed PEP 3123 to update the spelling.

> FWIW, should PyUnicode_Check, PyTuple_Check etc. also change to
> PyUnicode_CHECK, PyTuple_CHECK etc because they are macros?

While this would technically be the right thing to do, those macros
have had a long and productive life before 2.6 so I'd rather be
compatible. IOW no.

New changeset 63183596db79 by Gregory P. Smith in branch '2.7':
Per issue21949 and issue1629: Document the Py_SIZE, Py_TYPE, Py_REFCNT macros.
https://hg.python.org/cpython/rev/63183596db79