mail failed; [SSL: UNKNOWN_PROTOCOL] unknown protocol (_ssl.c:645)

This is a question about sending an email through an authenticated SMTP (not gmail). The below script was put together through various questions and answers on this site but I get an error that has no "googlable" candidates for this particular combination of tools. I'm working in Python 3.5.1 and it produces this error:

mail failed; [SSL: UNKNOWN_PROTOCOL] unknown protocol (_ssl.c:645)

Is this client side error or server? Am I missing some certificates I'm not aware of? AFAIK server supports SSL authentication. Any thoughts and nudges in the right direction will be appreciated.

import sys
from smtplib import SMTP_SSL as SMTP
from email.mime.text import MIMEText

# credentials masked, obviously
SMTPserver = 'myserver'
sender = 'mymail'
destination = ['recipient']

USERNAME = "myusername"
PASSWORD = "mypass"

# typical values for text_subtype are plain, html, xml
text_subtype = 'plain'

content = """\
Test message
"""

subject = "Sent from Python"

try:
    msg = MIMEText(content, text_subtype)
    msg['Subject'] = subject
    msg['From'] = sender
    conn = SMTP(host=SMTPserver, port=465)
    conn.set_debuglevel(False)
    conn.login(USERNAME, PASSWORD)

    try:
        conn.sendmail(sender, destination, msg.as_string())
    finally:
        conn.quit()

except Exception as exc:
    sys.exit("mail failed; %s" % str(exc))