Status:

* Py_SET_REFCNT(), Py_SET_TYPE() and Py_SET_SIZE() functions added to Python 3.9.
* Python and Cython have been modified to use Py_TYPE(), Py_SET_REFCNT(), Py_IS_TYPE(), etc.
* pythoncapi_compat.h header file has been created to provide new functions to Python 3.6: https://github.com/pythoncapi/pythoncapi_compat
* Script has been created to upgrade C extensions to add support for Python 3.10 without losing support for old Python versions: https://github.com/pythoncapi/pythoncapi_compat
* PEP 620 "Hide implementation details from the C API" written
* Py_TYPE() and Py_SIZE() were converted to a static inline function to deny "Py_TYPE(obj) = type;" syntax, but this change has been reverted during Python 3.10 development cycle since it broke too many C extension modules. (msg381337)

TODO:

* Maybe add a new formatter for type names (msg361523)
* Avoid sizeof(PyObject) in PyType_FromSpec() (msg366473)

The purpose of this issue is only to fix the API part. Replacing static inline functions with opaque function calls (stable ABI) is not in the scope of this issue.