available-ips endpoint is broken when using VRFs

[fabi125]

NetBox version

3.0-dev

Python version

3.8

Steps to Reproduce

1. Go to https://beta-demo.netbox.dev/
2. Create a VRF
3. Create a prefix inside that VRF
4. Go to https://beta-demo.netbox.dev/api/ipam/prefixes/id/available-ips/

Expected Behavior

List of available IPs is returned.

Observed Behavior

{"error": "Object of type VRF is not JSON serializable", "exception": "TypeError", "netbox_version": "3.0-dev", "python_version": "3.8.10"}

This is what the prefix looks like:

{
    "id": 73,
    "url": "https://beta-demo.netbox.dev/api/ipam/prefixes/73/",
    "display": "10.0.0.0/24",
    "family": {
        "value": 4,
        "label": "IPv4"
    },
    "prefix": "10.0.0.0/24",
    "site": null,
    "vrf": {
        "id": 1,
        "url": "https://beta-demo.netbox.dev/api/ipam/vrfs/1/",
        "display": "Test VRF",
        "name": "Test VRF",
        "rd": null
    },
    "tenant": null,
    "vlan": null,
    "status": {
        "value": "active",
        "label": "Active"
    },
    "role": null,
    "is_pool": false,
    "mark_utilized": false,
    "description": "",
    "tags": [],
    "custom_fields": {},
    "created": "2021-08-05",
    "last_updated": "2021-08-05T11:51:42.530747Z",
    "children": 0,
    "_depth": 0
}