why prompted ModuleNotFoundError: No module named '_ssl' when I call python3 test.py on the version python3.7

问题

>>> import flask
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/ENV/lib/python3.7/site-packages/flask/__init__.py", 
line 21, in <module>
from .app import Flask, Request, Response
  File "/home/ENV/lib/python3.7/site-packages/flask/app.py", line 25, in <module>
from . import cli, json
  File "/home/ENV/lib/python3.7/site-packages/flask/cli.py", line 18, in <module>
import ssl
  File "/usr/local/python3/lib/python3.7/ssl.py", line 98, in <module>
import _ssl             # if we can't import it, let the error propagate
ModuleNotFoundError: No module named '_ssl'
>>> 

Why prompted " ModuleNotFoundError: No module named '_ssl'" ?

Python3.7 OS:Ubuntu

来源：https://stackoverflow.com/questions/56072646/why-prompted-modulenotfounderror-no-module-named-ssl-when-i-call-python3-tes