$ openssl req -new -newkey rsa:3072 -sha256 -x509 -days 365 -nodes -out certkey.pem -keyout certkey.pem -subj "/O=selfsigned"
$ cat test.py
import ssl
ctx = ssl.SSLContext(ssl.PROTOCOL_TLSv1)
ctx.load_cert_chain('certkey.pem', None)
$ python2.7 test.py
Traceback (most recent call last):
  File "test.py", line 3, in <module>
    ctx.load_cert_chain('certkey.pem', None)
TypeError: coercing to Unicode: need string or buffer, NoneType found
$ python3 test.py