dont reassign os.environ

os.environ is an `environ` object, not a dict. by only interacting with
it through builtin functions we can ensure it remains properly
accessible to third party libraries which might interact with it in
different ways.

See https://github.com/alphagov/notifications-api/commit/d2441466 for
more detail

tests/app/test_config.py

@@ -12,21 +12,24 @@ def cf_conf():
 
 
 @pytest.fixture
-def reload_config():
+def reload_config(os_environ):
     """
     Reset config, by simply re-running config.py from a fresh environment
     """
     old_env = os.environ.copy()
+    os.environ.clear()
 
     yield
 
-    os.environ = old_env
+    os.environ.clear()
+    for k, v in old_env.items():
+        os.environ[k] = v
     importlib.reload(config)
 
 
-def test_load_cloudfoundry_config_if_available(monkeypatch, reload_config):
+def test_load_cloudfoundry_config_if_available(reload_config):
     os.environ['API_HOST_NAME'] = 'env'
-    monkeypatch.setenv('VCAP_APPLICATION', 'some json blob')
+    os.environ['VCAP_APPLICATION'] = 'some json blob'
 
     with mock.patch('app.cloudfoundry_config.extract_cloudfoundry_config', side_effect=cf_conf) as cf_config:
         # reload config so that its module level code (ie: all of it) is re-instantiated
@@ -38,10 +41,10 @@ def test_load_cloudfoundry_config_if_available(monkeypatch, reload_config):
     assert config.Config.API_HOST_NAME == 'cf'
 
 
-def test_load_config_if_cloudfoundry_not_available(monkeypatch, reload_config):
+def test_load_config_if_cloudfoundry_not_available(reload_config):
     os.environ['API_HOST_NAME'] = 'env'
 
-    monkeypatch.delenv('VCAP_APPLICATION', raising=False)
+    os.environ.pop('VCAP_APPLICATION', None)
 
     with mock.patch('app.cloudfoundry_config.extract_cloudfoundry_config') as cf_config:
         # reload config so that its module level code (ie: all of it) is re-instantiated