port set --security-group: 'Port' object has no attribute 'security_groups'

Situation:
```
(openstack) port list --long
+--------------------------------------+------------------------------------------+-------------------+-----------------------------------------------------+--------+-----------------------------------------------------+--------------------------+
| ID | Name | MAC Address | Fixed IP Addresses | Status | Security Groups | Device Owner |
+--------------------------------------+------------------------------------------+-------------------+-----------------------------------------------------+--------+-----------------------------------------------------+--------------------------+
| 1ff97542-ab3b-4f91-b8b8-b113f7443052 | port2 | fa:16:3e:06:1d:10 | ip_address='192.168.1.14', subnet_id='8c9c6652 | ACTIVE | 721f6ebc-8eb3-42da-9a73-78d7cb30a45f | |
| | | | -b87c-46ed-aaab-2982cbe76aa2' | | | |
```

Error:
```
(openstack) port set --security-group c9ee9216-8d74-4e73-91d9-26a80e3620ca 1ff97542-ab3b-4f91-b8b8-b113f7443052
'Port' object has no attribute 'security_groups'
```

What I would expect: No "has no attribute" error should appear, as this flag is even documented:
```
(openstack) port set -h
usage: port set [-h] [--description <description>] [--device <device-id>]
                [--device-owner <device-owner>] [--vnic-type <vnic-type>]
                [--host <host-id>] [--dns-name dns-name]
                [--enable | --disable] [--name <name>]
                [--fixed-ip subnet=<subnet>,ip-address=<ip-address>]
                [--no-fixed-ip] [--binding-profile <binding-profile>]
                [--no-binding-profile] [--security-group <security-group>]
                [--no-security-group]
                [--enable-port-security | --disable-port-security]
                <port>
[...]
  --security-group <security-group>
                        Security group to associate with this port (name or
                        ID) (repeat option to set multiple security groups)
```