chore: drop use of pytz in systests

See #479 for rationale.

tests/system/test_backup_api.py

@@ -16,7 +16,6 @@
 import time
 
 import pytest
-from pytz import UTC
 
 from google.api_core import exceptions
 from google.cloud import spanner_v1
@@ -82,7 +81,7 @@ def diff_config_instance(
 
 @pytest.fixture(scope="session")
 def database_version_time():
-    return datetime.datetime.utcnow().replace(tzinfo=UTC)
+    return datetime.datetime.utcnow().replace(tzinfo=datetime.timezone.utc)
 
 
 @pytest.fixture(scope="session")
@@ -126,7 +125,7 @@ def test_backup_workflow(
 
     backup_id = _helpers.unique_id("backup_id", separator="_")
     expire_time = datetime.datetime.utcnow() + datetime.timedelta(days=3)
-    expire_time = expire_time.replace(tzinfo=UTC)
+    expire_time = expire_time.replace(tzinfo=datetime.timezone.utc)
     encryption_enum = CreateBackupEncryptionConfig.EncryptionType
     encryption_config = CreateBackupEncryptionConfig(
         encryption_type=encryption_enum.GOOGLE_DEFAULT_ENCRYPTION,
@@ -164,7 +163,7 @@ def test_backup_workflow(
 
     # Update with valid argument.
     valid_expire_time = datetime.datetime.utcnow() + datetime.timedelta(days=7)
-    valid_expire_time = valid_expire_time.replace(tzinfo=UTC)
+    valid_expire_time = valid_expire_time.replace(tzinfo=datetime.timezone.utc)
     backup.update_expire_time(valid_expire_time)
     assert valid_expire_time == backup.expire_time
 
@@ -202,7 +201,7 @@ def test_backup_create_w_version_time_dflt_to_create_time(
 ):
     backup_id = _helpers.unique_id("backup_id", separator="_")
     expire_time = datetime.datetime.utcnow() + datetime.timedelta(days=3)
-    expire_time = expire_time.replace(tzinfo=UTC)
+    expire_time = expire_time.replace(tzinfo=datetime.timezone.utc)
 
     # Create backup.
     backup = shared_instance.backup(
@@ -229,7 +228,7 @@ def test_backup_create_w_version_time_dflt_to_create_time(
 
 def test_backup_create_w_invalid_expire_time(shared_instance, shared_database):
     backup_id = _helpers.unique_id("backup_id", separator="_")
-    expire_time = datetime.datetime.utcnow().replace(tzinfo=UTC)
+    expire_time = datetime.datetime.utcnow().replace(tzinfo=datetime.timezone.utc)
 
     backup = shared_instance.backup(
         backup_id, database=shared_database, expire_time=expire_time
@@ -245,9 +244,9 @@ def test_backup_create_w_invalid_version_time_past(
 ):
     backup_id = _helpers.unique_id("backup_id", separator="_")
     expire_time = datetime.datetime.utcnow() + datetime.timedelta(days=3)
-    expire_time = expire_time.replace(tzinfo=UTC)
+    expire_time = expire_time.replace(tzinfo=datetime.timezone.utc)
     version_time = datetime.datetime.utcnow() - datetime.timedelta(days=10)
-    version_time = version_time.replace(tzinfo=UTC)
+    version_time = version_time.replace(tzinfo=datetime.timezone.utc)
 
     backup = shared_instance.backup(
         backup_id,
@@ -266,9 +265,9 @@ def test_backup_create_w_invalid_version_time_future(
 ):
     backup_id = _helpers.unique_id("backup_id", separator="_")
     expire_time = datetime.datetime.utcnow() + datetime.timedelta(days=3)
-    expire_time = expire_time.replace(tzinfo=UTC)
+    expire_time = expire_time.replace(tzinfo=datetime.timezone.utc)
     version_time = datetime.datetime.utcnow() + datetime.timedelta(days=2)
-    version_time = version_time.replace(tzinfo=UTC)
+    version_time = version_time.replace(tzinfo=datetime.timezone.utc)
 
     backup = shared_instance.backup(
         backup_id,
@@ -291,7 +290,7 @@ def test_database_restore_to_diff_instance(
 ):
     backup_id = _helpers.unique_id("backup_id", separator="_")
     expire_time = datetime.datetime.utcnow() + datetime.timedelta(days=3)
-    expire_time = expire_time.replace(tzinfo=UTC)
+    expire_time = expire_time.replace(tzinfo=datetime.timezone.utc)
 
     # Create backup.
     backup = shared_instance.backup(
@@ -325,7 +324,7 @@ def test_multi_create_cancel_update_error_restore_errors(
     backup_id_1 = _helpers.unique_id("backup_id1", separator="_")
     backup_id_2 = _helpers.unique_id("backup_id2", separator="_")
     expire_time = datetime.datetime.utcnow() + datetime.timedelta(days=3)
-    expire_time = expire_time.replace(tzinfo=UTC)
+    expire_time = expire_time.replace(tzinfo=datetime.timezone.utc)
 
     backup1 = shared_instance.backup(
         backup_id_1, database=shared_database, expire_time=expire_time
@@ -357,7 +356,7 @@ def test_multi_create_cancel_update_error_restore_errors(
     # Update expire time to invalid value.
     max_expire_days = 366  # documented maximum
     invalid_expire_time = datetime.datetime.now().replace(
-        tzinfo=UTC
+        tzinfo=datetime.timezone.utc
     ) + datetime.timedelta(days=max_expire_days + 1)
     with pytest.raises(exceptions.InvalidArgument):
         backup1.update_expire_time(invalid_expire_time)
@@ -388,7 +387,7 @@ def test_instance_list_backups(
     backup_id_2 = _helpers.unique_id("backup_id2", separator="_")
 
     expire_time_1 = datetime.datetime.utcnow() + datetime.timedelta(days=21)
-    expire_time_1 = expire_time_1.replace(tzinfo=UTC)
+    expire_time_1 = expire_time_1.replace(tzinfo=datetime.timezone.utc)
     expire_time_1_stamp = expire_time_1.strftime("%Y-%m-%dT%H:%M:%S.%fZ")
 
     backup1 = shared_instance.backup(
@@ -399,7 +398,7 @@ def test_instance_list_backups(
     )
 
     expire_time_2 = datetime.datetime.utcnow() + datetime.timedelta(days=1)
-    expire_time_2 = expire_time_2.replace(tzinfo=UTC)
+    expire_time_2 = expire_time_2.replace(tzinfo=datetime.timezone.utc)
     backup2 = shared_instance.backup(
         backup_id_2, database=second_database, expire_time=expire_time_2
     )
@@ -410,7 +409,9 @@ def test_instance_list_backups(
     op1.result()  # blocks indefinitely
     backup1.reload()
 
-    create_time_compare = datetime.datetime.utcnow().replace(tzinfo=UTC)
+    create_time_compare = datetime.datetime.utcnow().replace(
+        tzinfo=datetime.timezone.utc
+    )
     create_time_stamp = create_time_compare.strftime("%Y-%m-%dT%H:%M:%S.%fZ")
 
     backup2.create()