For Python < 3.9 the tp_as_buffer member is set explicitly if the
interface has a bf_getbuffer slot defined. This fixes#3211.
Enabled buffer interface for non-builtin test.
This only works with Python >= 3.12, where methods __buffer__ and
__release_buffer__ were added. Unfortunately it's not practical for
these methods to reuse the slot methods (or vice versa).
Disable Py_LIMITED_API if below 3.11. The Py_buffer struct and
associated functions are not defined in earlier stable API versions.
Closes#3211
Julien Schueller