Invalid response for oauth token

Getting the following error following update to the latest release (4.0.0)

```
Invalid response - 400 for https://eu.battle.net/oauth/token?grant_type=client_credentials&client_id=REDACTED&client_secret=REDACTED
```

with:

```
wow_api_client.get_guild_roster(GAME_REGION, f"profile-{GAME_REGION}", GAME_REALM, GAME_GUILD)
```