client: Use file as a context manager.

caucase/client.py

@@ -110,7 +110,8 @@ class CaucaseClient(object):
     Return whether an update happened.
     """
     if os.path.exists(crl_path):
-      my_crl = utils.load_crl(open(crl_path, 'rb').read(), ca_list)
+      with open(crl_path, 'rb') as crl_file:
+        my_crl = utils.load_crl(crl_file.read(), ca_list)
     else:
       my_crl = None
     latest_crl_pem = cls(ca_url=url).getCertificateRevocationList()