如何从SSLContext获取SSL证书的有效期？

import ssl

context = ssl.create_default_context()

import socket

hostname = 'www.example.com'
port = 443

with socket.create_connection((hostname, port)) as sock:
    with context.wrap_socket(sock, server_hostname=hostname) as ssock:
        cert = ssock.getpeercert()

import datetime

not_before = datetime.datetime.strptime(cert['notBefore'], "%b %d %H:%M:%S %Y %Z")
not_after = datetime.datetime.strptime(cert['notAfter'], "%b %d %H:%M:%S %Y %Z")

print("证书有效期开始时间：", not_before)
print("证书有效期结束时间：", not_after)