[1.5.x] Replaced deprecated sslerror by ssl.SSLError

The exact conditions on which this exception is raised are not
known, but this replacement is the best guess we can do at this
point.
Backport of 850630b4b7 from master.

django/core/mail/backends/smtp.py

@@ -1,6 +1,6 @@
 """SMTP email backend class."""
 import smtplib
-import socket
+import ssl
 import threading
 
 from django.conf import settings
@@ -65,7 +65,7 @@ class EmailBackend(BaseEmailBackend):
         try:
             try:
                 self.connection.quit()
-            except (socket.sslerror, smtplib.SMTPServerDisconnected):
+            except (ssl.SSLError, smtplib.SMTPServerDisconnected):
                 # This happens when calling quit() on a TLS connection
                 # sometimes, or when the connection was already disconnected
                 # by the server.