Use PyThreadState_EnterTracing()

[vstinner]

Add a copy of the pythoncapi_compat.h header:
https://github.com/pythoncapi/pythoncapi_compat

pythoncapi_compat.h provides PyThreadState_EnterTracing() and
PyThreadState_LeaveTracing() to Python 3.10 and older. They function
were added to Python 3.11.0a2:
python/cpython@547d26a

PyThreadState.cframe.use_tracing format changed again in Python
3.11.0a2 (value set to 0 or 255):
python/cpython@bd627eb

This change makes greenlet ready for these changes and future CPython
internal changes.

[vstinner]

See https://bugs.python.org/issue43760 discussion for recent CPython changes and discussions about adding new C API functions to abstract accesses to PyThreadState and PyFrameObject.

I'm the author and maintainer of pythoncapi_compat. Using it, it becomes to remove the following lines from src/greenlet/greenlet_cpython_compat.hpp:

#if PY_VERSION_HEX < 0x03090000
// The official version only became available in 3.9
#    define PyObject_GC_IsTracked(o) _PyObject_GC_IS_TRACKED(o)
#endif

But I suggest to do it in a separated change, to keep this PR as small as possible.

I wrote pythoncapi_compat to help C extension maintainers to keep up with recent Python C API changes.

[jamadden]

Thanks for the heads up!

Rather than vendor an entire header file, I'll probably just add only the definitions greenlet needs to the existing header as it needs them.

[vstinner]

Oh, pythoncapi_compat.h would also allow to remove another compatibility function:

#ifndef Py_SET_REFCNT
/* Py_REFCNT and Py_SIZE macros are converted to functions
https://bugs.python.org/issue39573 */
#    define Py_SET_REFCNT(obj, refcnt) Py_REFCNT(obj) = (refcnt)
#endif

[vstinner]

Rather than vendor an entire header file, I'll probably just add only the definitions greenlet needs to the existing header as it needs them.

My long term plan is to ship pythoncapi_compat.h as part of Python itself and/or install it on the system, as the six module is now preinstalled or easy to install on any system. For now, I prefer to copy it in each project.

[vstinner]

If you don't want to use pythoncapi_compat.h for now, here is a different PR without it: #268

[jamadden]

Fixed by #268