postgres data-observability - fix lint errors in test_data_observability.py

Newer ddev catches three issues older ddev missed: misplaced calendar/datetime
imports below an inline comment block, plus two unused locals (tick_time,
next_run). Moves the imports to the top of the file and removes the unused
assignments.

Signed-off-by: mobuchowski <maciej.obuchowski@datadoghq.com>

Author: mobuchowski

postgres/tests/test_data_observability.py

@@ -3,6 +3,8 @@
 # Licensed under a 3-clause BSD style license (see LICENSE)
 from __future__ import annotations
 
+import calendar
+import datetime
 import json
 from contextlib import contextmanager
 from copy import deepcopy
@@ -24,9 +26,6 @@
 #
 # We use an absolute epoch so croniter's calendar arithmetic stays stable.
 # 2026-01-01 00:49:00 UTC  →  1751331940  (approx; exact value computed below)
-import calendar
-import datetime
-
 _BASE_EPOCH = calendar.timegm(datetime.datetime(2026, 1, 1, 0, 49, 0).timetuple())  # 00:49:00 UTC
 
 pytestmark = pytest.mark.unit
@@ -698,7 +697,6 @@ def test_neither_set_skipped(pg_instance, aggregator):
 def test_lateness_metric_emitted_for_cron(pg_instance, aggregator, monkeypatch):
     """Cron query fired late emits a positive lateness gauge with mode:cron tag."""
     # next_run will be at 00:50:00; fire at 00:52:00 → 120s lateness
-    tick_time = float(_BASE_EPOCH + 60)  # 00:50:00 exactly
     fire_time = float(_BASE_EPOCH + 180)  # 00:52:00
 
     call_count = [0]
@@ -785,7 +783,6 @@ def test_lateness_clamped_at_zero(pg_instance, aggregator, monkeypatch):
     # First call: registers next_run = 01:50:00 (future)
     check.data_observability.run_job()
     mid = CRON_QUERY['monitor_id']
-    next_run = check.data_observability._next_run[mid]
 
     # Manually set next_run to a value slightly in the future to simulate skew
     # (i.e., scheduled_fire_time > now_at_fire_start)