py/vm: Don't add traceback info for exc's propagated through a finally.

With this patch exception tracebacks that go through a finally are improved
(less confusing, match CPython), and it makes finally's slightly more
efficient (in time and RAM) because they no longer need to add a traceback.

Partially fixes issue #2928.

tests/misc/print_exception.py

@@ -47,6 +47,16 @@ except Exception as e:
     print('caught')
     print_exc(e)
 
+# Test that an exception propagated through a finally doesn't have a traceback added there
+try:
+    try:
+        f()
+    finally:
+        print('finally')
+except Exception as e:
+    print('caught')
+    print_exc(e)
+
 # Here we have a function with lots of bytecode generated for a single source-line, and
 # there is an error right at the end of the bytecode.  It should report the correct line.
 def f():