fix: force PORT to be a string in concatenation

Author: christian-roggia

Diff for: aip_site/server.py

@@ -92,5 +92,5 @@ def fx():
         # This is the dev server, so plow over whatever the configuration
         # says that the site URL is.
         flask.g.site.config.setdefault('urls', {})
-        flask.g.site.config['urls']['site'] = 'http://localhost:' + os.environ.get('PORT', 4000)
+        flask.g.site.config['urls']['site'] = 'http://localhost:' + str(os.environ.get('PORT', 4000))
     return fx