Fix error using the website URL setting

Author: nikdoof

hackspaceapi/services/website.py

@@ -23,7 +23,7 @@ def get_membership_data() -> Optional[Iterable]:
     Pull the JSON formatted membership plan data from the Leigh Hackspace
     website.
     """
-    url = urljoin(settings.hackspace_website_url, "/membership/index.json")
+    url = urljoin(str(settings.hackspace_website_url), "/membership/index.json")
     try:
         resp = session.get(url)
         if resp.ok: