fix smtp compatibility with python 3.12 (#54)

* remove custom tls settings

* Update smtp.py

flask_mailman/backends/smtp.py

@@ -76,7 +76,13 @@ def open(self):
             # TLS/SSL are mutually exclusive, so only attempt TLS over
             # non-secure connections.
             if not self.use_ssl and self.use_tls:
-                self.connection.starttls(keyfile=self.ssl_keyfile, certfile=self.ssl_certfile)
+                if self.ssl_certfile:
+                    context = ssl.SSLContext().load_cert_chain(
+                        self.ssl_certfile, keyfile=self.ssl_keyfile
+                    )
+                else:
+                    context = None
+                self.connection.starttls(context=context)
             if self.username and self.password:
                 self.connection.login(self.username, self.password)
             return True