Stop using datetime.utcnow() in tests

datetime.utcnow() is deprecated for Python 3.12+, and raises a warning.
Since warnings are treated as errors, this results in test failures.
Since utcnow calls are done by the SQLAlchemy mapping machinery, we need
to use a callable.

Fixes pallets-eco#1303

tests/test_model.py

@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 import typing as t
-from datetime import datetime
+from datetime import datetime, timezone
 
 import pytest
 import sqlalchemy as sa
@@ -14,6 +14,11 @@
 from flask_sqlalchemy.model import Model
 
 
+class UTCNow(datetime):
+    def __new__(self):
+        return datetime.now(tz=timezone.utc)
+
+
 def test_default_model_class_1x(app: Flask) -> None:
     db = SQLAlchemy(app)
 
@@ -147,12 +152,12 @@ def test_abstractmodel(app: Flask, model_class: t.Any) -> None:
         class TimestampModel(db.Model):
             __abstract__ = True
             created: sa_orm.Mapped[datetime] = sa_orm.mapped_column(
-                db.DateTime, nullable=False, insert_default=datetime.utcnow, init=False
+                db.DateTime, nullable=False, insert_default=UTCNow, init=False
             )
             updated: sa_orm.Mapped[datetime] = sa_orm.mapped_column(
                 db.DateTime,
-                insert_default=datetime.utcnow,
-                onupdate=datetime.utcnow,
+                insert_default=UTCNow,
+                onupdate=UTCNow,
                 init=False,
             )
 
@@ -167,10 +172,10 @@ class Post(TimestampModel):
         class TimestampModel(db.Model):  # type: ignore[no-redef]
             __abstract__ = True
             created: sa_orm.Mapped[datetime] = sa_orm.mapped_column(
-                db.DateTime, nullable=False, default=datetime.utcnow
+                db.DateTime, nullable=False, default=UTCNow
             )
             updated: sa_orm.Mapped[datetime] = sa_orm.mapped_column(
-                db.DateTime, default=datetime.utcnow, onupdate=datetime.utcnow
+                db.DateTime, default=UTCNow, onupdate=UTCNow
             )
 
         class Post(TimestampModel):  # type: ignore[no-redef]
@@ -181,9 +186,9 @@ class Post(TimestampModel):  # type: ignore[no-redef]
 
         class TimestampModel(db.Model):  # type: ignore[no-redef]
             __abstract__ = True
-            created = db.Column(db.DateTime, nullable=False, default=datetime.utcnow)
+            created = db.Column(db.DateTime, nullable=False, default=UTCNow)
             updated = db.Column(
-                db.DateTime, onupdate=datetime.utcnow, default=datetime.utcnow
+                db.DateTime, onupdate=UTCNow, default=UTCNow
             )
 
         class Post(TimestampModel):  # type: ignore[no-redef]
@@ -207,12 +212,12 @@ def test_mixinmodel(app: Flask, model_class: t.Any) -> None:
 
         class TimestampMixin(sa_orm.MappedAsDataclass):
             created: sa_orm.Mapped[datetime] = sa_orm.mapped_column(
-                db.DateTime, nullable=False, insert_default=datetime.utcnow, init=False
+                db.DateTime, nullable=False, insert_default=UTCNow, init=False
             )
             updated: sa_orm.Mapped[datetime] = sa_orm.mapped_column(
                 db.DateTime,
-                insert_default=datetime.utcnow,
-                onupdate=datetime.utcnow,
+                insert_default=UTCNow,
+                onupdate=UTCNow,
                 init=False,
             )
 
@@ -226,10 +231,10 @@ class Post(TimestampMixin, db.Model):
 
         class TimestampMixin:  # type: ignore[no-redef]
             created: sa_orm.Mapped[datetime] = sa_orm.mapped_column(
-                db.DateTime, nullable=False, default=datetime.utcnow
+                db.DateTime, nullable=False, default=UTCNow
             )
             updated: sa_orm.Mapped[datetime] = sa_orm.mapped_column(
-                db.DateTime, default=datetime.utcnow, onupdate=datetime.utcnow
+                db.DateTime, default=UTCNow, onupdate=UTCNow
             )
 
         class Post(TimestampMixin, db.Model):  # type: ignore[no-redef]
@@ -239,9 +244,9 @@ class Post(TimestampMixin, db.Model):  # type: ignore[no-redef]
     else:
 
         class TimestampMixin:  # type: ignore[no-redef]
-            created = db.Column(db.DateTime, nullable=False, default=datetime.utcnow)
+            created = db.Column(db.DateTime, nullable=False, default=UTCNow)
             updated = db.Column(
-                db.DateTime, onupdate=datetime.utcnow, default=datetime.utcnow
+                db.DateTime, onupdate=UTCNow, default=UTCNow
             )
 
         class Post(TimestampMixin, db.Model):  # type: ignore[no-redef]