python: Migrate open statements to use with.

This is low priority, but it's nice to be consistently using the best
practice pattern.

Fixes: #12419.

puppet/zulip/files/nagios_plugins/zulip_app_frontend/check_cron_file

@@ -18,7 +18,8 @@ def nagios_from_file(results_file: str, max_time_diff: int=60 * 2) -> 'Tuple[int
     This file is created by various nagios checking cron jobs such as
     check-rabbitmq-queues and check-rabbitmq-consumers"""
 
-    data = open(results_file).read().strip()
+    with open(results_file, 'r') as f:
+        data = f.read().strip()
     pieces = data.split('|')
 
     if not len(pieces) == 4:

puppet/zulip_ops/files/nagios_plugins/zulip_zephyr_mirror/check_personal_zephyr_mirrors

@@ -34,7 +34,8 @@ down_count = 0
 for results_file_name in os.listdir(RESULTS_DIR):
     this_state = "OK"
     results_file = os.path.join(RESULTS_DIR, results_file_name)
-    data = open(results_file).read().strip()
+    with open(results_file, 'r') as f:
+        data = f.read().strip()
     last_check = os.stat(results_file).st_mtime
     time_since_last_check = time.time() - last_check
     # time_since_last_check threshold needs to be strictly greater

puppet/zulip_ops/files/nagios_plugins/zulip_zephyr_mirror/check_zephyr_mirror

@@ -32,7 +32,8 @@ def report(state, data, last_check):
         data))
     exit(states[state])
 
-data = open(RESULTS_FILE).read().strip()
+with open(RESULTS_FILE, 'r') as f:
+    data = f.read().strip()
 if data.split("\n")[-1].strip() == "0":
     state = "OK"
 else: