Strip environment for test_settings

Certain variables, when set in the running environment, can make tests
fail. As environment variable names are case-insensitive by default,
it's better to start with a clean environment.

Issue: #463

tests/test_settings.py

@@ -7,6 +7,7 @@
 from enum import IntEnum
 from pathlib import Path
 from typing import Any, Callable, Dict, Generic, Hashable, List, Optional, Set, Tuple, Type, TypeVar, Union
+from unittest import mock
 
 import pytest
 from annotated_types import MinLen
@@ -68,6 +69,12 @@ class SettingWithPopulateByName(BaseSettings):
     model_config = SettingsConfigDict(populate_by_name=True)
 
 
+@pytest.fixture(autouse=True)
+def clean_env():
+    with mock.patch.dict(os.environ, clear=True):
+        yield
+
+
 def test_sub_env(env):
     env.set('apple', 'hello')
     s = SimpleSettings()