correctly close file after it's opened

src/common/jingle_xtls.py

@@ -114,8 +114,8 @@ def get_context(fingerprint, verify_cb=None):
 def send_cert(con, jid_from, sid):
     certpath = os.path.join(gajim.MY_CERT_DIR, SELF_SIGNED_CERTIFICATE) + \
         '.cert'
-    certfile = open(certpath, 'r')
     certificate = ''
+    with open(certpath, 'r') as certfile:
         for line in certfile.readlines():
             if not line.startswith('-'):
                 certificate += line