Move to nanobind 3.1 and release 8.0.1.3

The handle caster follows nanobind 3's renamed detail API (keep_alive_cb,
NB_CALL(nb_type_put), uint32_t caster flags). Bound surface unchanged per
sigdiff; list iteration ~4x faster.

scripts/_env.sh

@@ -24,10 +24,10 @@ if [ ! -x "$VENV/bin/python" ]; then
     "$VENV/bin/pip" install -q --upgrade pip
     # numpy is a test dependency only: the n3xd_ocp bulk APIs hand back
     # ndarrays, so their tests need it to check the results.
-    # nanobind 3 reworked the type-caster and rv_policy interfaces
-    # occt_handle.h builds on, so moving to it is a deliberate bump, not
-    # something a fresh cache picks up.
-    "$VENV/bin/pip" install -q build "nanobind<3" scikit-build-core pytest numpy
+    # occt_handle.h builds on nanobind's detail API, which moves between
+    # majors, so the next one is a deliberate bump, not something a fresh
+    # cache picks up.
+    "$VENV/bin/pip" install -q build "nanobind>=3.1,<4" scikit-build-core pytest numpy
 fi
 PY="$VENV/bin/python"
 export PY VENV CACHE