Debug build uses the same ABI as release build

Python now uses the same ABI when built in release and in debug mode. On Unix,

when Python is build in debug mode, it is now possible to load C extensions

built in release mode and C extensions built using the stable ABI.

Release build and debug build are now ABI compatible: the ``Py_DEBUG`` define

no longer implies the ``Py_TRACE_REFS`` define which introduces the only ABI

incompatibility. A new ``./configure --with-trace-refs`` build option is now

required to get ``Py_TRACE_REFS`` define which adds :func:`sys.getobjects`

function and :envvar:`PYTHONDUMPREFS` environment variable.

(Contributed by Victor Stinner in :issue:`36465`.)

On Unix, C extensions are no longer linked to libpython. It is now possible to

load a C extension built using a shared library Python

with a statically linked Python.

(Contributed by Victor Stinner in :issue:`21536`.)

On Unix, when Python is built in debug mode, import now also looks for C

extensions compiled in release mode and for C extensions compiled with the

stable ABI.

(Contributed by Victor Stinner in :issue:`36722`.)