bigfile/py: Starting from py3.12 current exception state is kept only in current_exception no longer using curexc_type and curexc_traceback

curexc_type and curexc_traceback can all be derived from current exception value.
See https://github.com/python/cpython/commit/feec49c40736 and
https://github.com/python/cpython/issues/101578 for details.

-> Rework the code to only use current_exception on py ≥ 3.12 similarly
to how we did for "handled" exception in 8071e2da (bigfile/py: Starting
from py3.11 exception state is kept only in exc_value no longer using
exc_type and exc_traceback.

/cc @tomo
/reviewed-by @levin.zimmermann, @jerome
/reviewed-on !43 + slapos!1863 (comment 260064)