Vendor EnvironmentVarGuard for Python 3.14 compatibility

- Replace try/except import chain with a vendored EnvironmentVarGuard
  class using unittest.mock.patch.dict(os.environ)
- Add tests for the vendored EnvironmentVarGuard

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: rtibblesbot

tests/testapp/tests/compat.py

@@ -1,11 +1,21 @@
-try:
-    # In the Python EOL GH workflows, we have to install backported version
-    # as the Docker container images we use does not have the test module installed.
-    from backports.test.support import EnvironmentVarGuard # noqa F401
-except ImportError:
-    try:
-        # For python >3.8 and <3.10
-        from test.support import EnvironmentVarGuard # noqa F401
-    except ImportError:
-        # In Python 3.10, this has been moved to test.support.os_helper
-        from test.support.os_helper import EnvironmentVarGuard # noqa F401
+import os
+from unittest.mock import patch as _patch
+
+
+class EnvironmentVarGuard:
+    """
+    Vendored replacement for the removed test.support EnvironmentVarGuard.
+    Uses unittest.mock.patch.dict(os.environ) under the hood.
+    Supports the context-manager-with-dict-assignment pattern:
+        with EnvironmentVarGuard() as env: env[k] = v
+    """
+
+    def __init__(self):
+        self._patcher = _patch.dict(os.environ)
+
+    def __enter__(self):
+        self._patcher.start()
+        return os.environ
+
+    def __exit__(self, *args):
+        self._patcher.stop()

tests/testapp/tests/test_compat.py

@@ -0,0 +1,37 @@
+import os
+import uuid
+
+from django.test import SimpleTestCase
+
+from .compat import EnvironmentVarGuard
+
+
+class EnvironmentVarGuardTestCase(SimpleTestCase):
+    """Tests for the vendored EnvironmentVarGuard."""
+
+    def test_sets_env_var_inside_context(self):
+        key = "MORANGO_TEST_" + uuid.uuid4().hex[:8]
+        with EnvironmentVarGuard() as env:
+            env[key] = "test_value"
+            self.assertEqual(os.environ[key], "test_value")
+
+    def test_reverts_env_var_on_exit(self):
+        key = "MORANGO_TEST_" + uuid.uuid4().hex[:8]
+        with EnvironmentVarGuard() as env:
+            env[key] = "test_value"
+        self.assertNotIn(key, os.environ)
+
+    def test_reverts_modified_env_var_on_exit(self):
+        key = "MORANGO_TEST_" + uuid.uuid4().hex[:8]
+        os.environ[key] = "original"
+        try:
+            with EnvironmentVarGuard() as env:
+                env[key] = "modified"
+                self.assertEqual(os.environ[key], "modified")
+            self.assertEqual(os.environ[key], "original")
+        finally:
+            os.environ.pop(key, None)
+
+    def test_returns_os_environ(self):
+        with EnvironmentVarGuard() as env:
+            self.assertIs(env, os.environ)