Should use OIDC metadata discovery

https://github.com/xero-github/xero-python-oauth-starter/blob/b3e48ce9d87d1515799eac602301e453b7ac937b/app.py#L16

We should make use of the `https://identity.xero.com/.well-known/openid-configuration` for the token URLs, in case they ever need to be changed. 