test_urls: Block waiting for connections in the pool

Avoid `Pool is closed` exceptions.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

scripts/test_urls.py

@@ -85,7 +85,7 @@ headers={
     }
 
 session = requests.Session()
-adapter = requests.adapters.HTTPAdapter(max_retries=3)
+adapter = requests.adapters.HTTPAdapter(max_retries=3, pool_block=True)
 session.mount('http://', adapter)
 session.mount('https://', adapter)