tests: Use freezegun for time mocking to fix pypy3 compatibility

Use freezegun for time mocking instead of manually patching the datetime
module, as it provides a more streamlined solution that works both
on CPython and on PyPy.  Unfortunately, due to differences between
the C datetime extension used by CPython, and the pure Python version
of datetime (used by PyPy, and as a fallback on CPython), there does not
seem to be a trivial way to mock time that would work with both
versions.

Fixes #2708

Signed-off-by: Michał Górny <mgorny@gentoo.org>

requirements/test.txt

@@ -5,3 +5,4 @@
 
 # coverage measurement
 coverage==7.6.1
+freezegun==1.5.1

tests/test_updater_top_level_update.py

@@ -11,7 +11,9 @@
 import unittest
 from datetime import timezone
 from typing import Iterable, Optional
-from unittest.mock import MagicMock, Mock, call, patch
+from unittest.mock import MagicMock, call, patch
+
+import freezegun
 
 from tests import utils
 from tests.repository_simulator import RepositorySimulator
@@ -306,8 +308,7 @@ def test_new_timestamp_unsigned(self) -> None:
 
         self._assert_files_exist([Root.type])
 
-    @patch.object(datetime, "datetime", wraps=datetime.datetime)
-    def test_expired_timestamp_version_rollback(self, mock_time: Mock) -> None:
+    def test_expired_timestamp_version_rollback(self) -> None:
         """Verifies that local timestamp is used in rollback checks even if it is expired.
 
         The timestamp updates and rollback checks are performed
@@ -331,19 +332,17 @@ def test_expired_timestamp_version_rollback(self, mock_time: Mock) -> None:
 
         self.sim.timestamp.version = 1
 
-        mock_time.now.return_value = datetime.datetime.now(
-            timezone.utc
-        ) + datetime.timedelta(days=18)
-        patcher = patch("datetime.datetime", mock_time)
+        patcher = freezegun.freeze_time(
+            datetime.datetime.now(timezone.utc) + datetime.timedelta(days=18)
+        )
         # Check that a rollback protection is performed even if
         # local timestamp has expired
         with patcher, self.assertRaises(BadVersionNumberError):
             self._run_refresh()
 
         self._assert_version_equals(Timestamp.type, 2)
 
-    @patch.object(datetime, "datetime", wraps=datetime.datetime)
-    def test_expired_timestamp_snapshot_rollback(self, mock_time: Mock) -> None:
+    def test_expired_timestamp_snapshot_rollback(self) -> None:
         """Verifies that rollback protection is done even if local timestamp has expired.
 
         The snapshot updates and rollback protection checks are performed
@@ -370,10 +369,9 @@ def test_expired_timestamp_snapshot_rollback(self, mock_time: Mock) -> None:
         self.sim.update_snapshot()
         self.sim.timestamp.expires = now + datetime.timedelta(days=21)
 
-        mock_time.now.return_value = datetime.datetime.now(
-            timezone.utc
-        ) + datetime.timedelta(days=18)
-        patcher = patch("datetime.datetime", mock_time)
+        patcher = freezegun.freeze_time(
+            datetime.datetime.now(timezone.utc) + datetime.timedelta(days=18)
+        )
         # Assert that rollback protection is done even if
         # local timestamp has expired
         with patcher, self.assertRaises(BadVersionNumberError):
@@ -736,8 +734,7 @@ def test_load_metadata_from_cache(self, wrapped_open: MagicMock) -> None:
         expected_calls = [("root", 2), ("timestamp", None)]
         self.assertListEqual(self.sim.fetch_tracker.metadata, expected_calls)
 
-    @patch.object(datetime, "datetime", wraps=datetime.datetime)
-    def test_expired_metadata(self, mock_time: Mock) -> None:
+    def test_expired_metadata(self) -> None:
         """Verifies that expired local timestamp/snapshot can be used for
         updating from remote.
 
@@ -761,10 +758,9 @@ def test_expired_metadata(self, mock_time: Mock) -> None:
 
         # Mocking time so that local timestam has expired
         # but the new timestamp has not
-        mock_time.now.return_value = datetime.datetime.now(
-            timezone.utc
-        ) + datetime.timedelta(days=18)
-        with patch("datetime.datetime", mock_time):
+        with freezegun.freeze_time(
+            datetime.datetime.now(timezone.utc) + datetime.timedelta(days=18)
+        ):
             self._run_refresh()
 
         # Assert that the final version of timestamp/snapshot is version 2