I changed the issue title to restrict its scope: "[C API] Avoid accessing PyObject and PyVarObject members directly: add Py_SET_TYPE() and Py_IS_TYPE(), disallow Py_TYPE(obj)=type".

Making PyObject and PyVarObject structures opaque is a broader topic which should be splited into sub-issues.

"Py_TYPE(obj)=type;" is now disallowed. I consider that the work of this issue is now completed and I close the issue.

Thanks everyone who help to fix these tedious issues!

You can continue to use this issue if you need my help to adapt your C extensions to Py_SET_TYPE()/Py_SET_SIZE().

See also the upgrade_pythoncapi.py script of the pythoncapi_compat project which helps to port your C extensions without losing support for old Python versions:
https://github.com/pythoncapi/pythoncapi_compat

See also the Py_TYPE() change announcement on the capi-sig list:
https://mail.python.org/archives/list/capi-sig@python.org/thread/WGRLTHTHC32DQTACPPX36TPR2GLJAFRB/