Remove optional install backports.zoneinfo for unsupported Python versions and associated code

setup.py

@@ -81,7 +81,7 @@ setup(
     author_email='tom@tomchristie.com',  # SEE NOTE BELOW (*)
     packages=find_packages(exclude=['tests*']),
     include_package_data=True,
-    install_requires=["django>=4.2", 'backports.zoneinfo;python_version<"3.9"'],
+    install_requires=["django>=4.2"],
     python_requires=">=3.9",
     zip_safe=False,
     classifiers=[

tests/test_fields.py

@@ -8,6 +8,7 @@ import warnings
 from decimal import ROUND_DOWN, ROUND_UP, Decimal
 from enum import auto
 from unittest.mock import patch
+from zoneinfo import ZoneInfo
 
 import pytest
 
@@ -30,11 +31,6 @@ from rest_framework.fields import (
 )
 from tests.models import UUIDForeignKeyTarget
 
-if sys.version_info >= (3, 9):
-    from zoneinfo import ZoneInfo
-else:
-    from backports.zoneinfo import ZoneInfo
-
 utc = datetime.timezone.utc
 
 # Tests for helper functions.