Use content type json constant (#40312)

tests/components/flo/test_config_flow.py

@@ -4,6 +4,7 @@ import time
 
 from homeassistant import config_entries, setup
 from homeassistant.components.flo.const import DOMAIN
+from homeassistant.const import CONTENT_TYPE_JSON
 
 from .common import TEST_EMAIL_ADDRESS, TEST_PASSWORD, TEST_TOKEN, TEST_USER_ID
 
@@ -53,7 +54,7 @@ async def test_form_cannot_connect(hass, aioclient_mock):
                 "timeNow": now,
             }
         ),
-        headers={"Content-Type": "application/json"},
+        headers={"Content-Type": CONTENT_TYPE_JSON},
         status=400,
     )
     result = await hass.config_entries.flow.async_init(