py/emitnative: Cancel caught exception once handled to prevent reraise.

The native emitter keeps the current exception in a slot in its C stack
(instead of on its Python value stack), so when it catches an exception it
must explicitly clear that slot so the same exception is not reraised later
on.

py/emitnative.c

@@ -1892,7 +1892,9 @@ STATIC void emit_native_pop_block(emit_t *emit) {
 }
 
 STATIC void emit_native_pop_except(emit_t *emit) {
-    (void)emit;
+    // Cancel any active exception so subsequent handlers don't see it
+    ASM_MOV_REG_IMM(emit->as, REG_TEMP0, (mp_uint_t)mp_const_none);
+    ASM_MOV_LOCAL_REG(emit->as, LOCAL_IDX_EXC_VAL(emit), REG_TEMP0);
 }
 
 STATIC void emit_native_unary_op(emit_t *emit, mp_unary_op_t op) {

tests/basics/try_finally1.py

@@ -69,3 +69,16 @@ try:  # top-level catch-all except to not fail script
 except:
     print("catch-all except")
 print()
+
+# case where a try-except within a finally cancels the exception
+print("exc-finally-subexcept")
+try:
+    print("try1")
+finally:
+    try:
+        print("try2")
+        foo
+    except:
+        print("except2")
+    print("finally1")
+print()