Added missing mine field to StackScript class (#271)

## 📝 Description

Added missing `mine` field to `StackScript` class

## ✔️ How to Test

`pytest test`

Ticket: TPT-1895

Author: ezilber-akamai-zz

linode_api4/objects/linode.py

@@ -1460,6 +1460,7 @@ class StackScript(Base):
         "deployments_total": Property(),
         "description": Property(mutable=True),
         "updated": Property(is_datetime=True),
+        "mine": Property(),
     }
 
     def _populate(self, json):

test/fixtures/linode_stackscripts_10079.json

@@ -0,0 +1,29 @@
+{
+    "created": "2018-01-01T00:01:01",
+    "deployments_active": 1,
+    "deployments_total": 12,
+    "description": "This StackScript installs and configures MySQL\n",
+    "id": 10079,
+    "images": [
+      "linode/debian9",
+      "linode/debian8"
+    ],
+    "is_public": true,
+    "label": "a-stackscript",
+    "mine": true,
+    "rev_note": "Set up MySQL",
+    "script": "\"#!/bin/bash\"\n",
+    "updated": "2018-01-01T00:01:01",
+    "user_defined_fields": [
+      {
+        "default": null,
+        "example": "hunter2",
+        "label": "Enter the password",
+        "manyOf": "avalue,anothervalue,thirdvalue",
+        "name": "DB_PASSWORD",
+        "oneOf": "avalue,anothervalue,thirdvalue"
+      }
+    ],
+    "user_gravatar_id": "a445b305abda30ebc766bc7fda037c37",
+    "username": "myuser"
+}

test/objects/linode_test.py

@@ -1,7 +1,7 @@
 from datetime import datetime
 from test.base import ClientBaseCase
 
-from linode_api4.objects import Config, Disk, Image, Instance, Type
+from linode_api4.objects import Config, Disk, Image, Instance, StackScript, Type
 
 
 class LinodeTest(ClientBaseCase):
@@ -452,6 +452,27 @@ def test_get_config(self):
         self.assertIsNotNone(config.devices)
 
 
+class StackScriptTest(ClientBaseCase):
+    """
+    Tests the methods of the StackScript class.
+    """
+
+    def test_get_stackscript(self):
+        """
+        Tests that a stackscript is loaded correctly by ID
+        """
+        stackscript = StackScript(self.client, 10079)
+
+        self.assertEqual(stackscript.id, 10079)
+        self.assertEqual(stackscript.deployments_active, 1)
+        self.assertEqual(stackscript.deployments_total, 12)
+        self.assertEqual(stackscript.rev_note, "Set up MySQL")
+        self.assertTrue(stackscript.mine)
+        self.assertTrue(stackscript.is_public)
+        self.assertIsNotNone(stackscript.user_defined_fields)
+        self.assertIsNotNone(stackscript.images)
+
+
 class TypeTest(ClientBaseCase):
     def test_get_types(self):
         """