Reduce the size of plain Python objects from 8 to 6 machine words.

[markshannon]

Currently a plain Python object has the following fields:

• values
• (managed) __dict__
• GC 1
• GC 2
• refcount
• __class__
• __weakrefs__
• WASTED

The WASTED field is present because, due to alignment, we must allocate a multiple of 2 words.
We can reduce this to 6 by merging the values and __dict__ pointers into one (only one can be valid at any time),
and then moving the __weakrefs__ field into the empty space.

• __weakrefs__
• values or __dict__
• GC 1
• GC 2
• refcount
• __class__

See faster-cpython/ideas#125 for full discussion

Linked PRs

• gh-95245: Document Py_TPFLAGS_PREHEADER #135861