Traceback (most recent call last):
File "main.py", line 2, in <module>
from vnpy.trader.engine import MainEngine
File "/opt/anaconda3/envs/py37/lib/python3.7/site-packages/vnpy/trader/engine.py", line 16, in <module>
from .app import BaseApp
ImportError: cannot import name 'BaseApp' from 'vnpy.trader.app' (/opt/anaconda3/envs/py37/lib/python3.7/site-packages/vnpy/trader/app/init.py)