Stups-tokens

- require a recent version of the `requests` library

- 6: set proper User-Agent HTTP header

- 5: add default connect and socket timeout (1.25 / 2.25 seconds)

- 4: add flag `ignore_expiration` to allow using expired tokens in failure scenarios (e.g. if the resource server implements token caching)

- 2: more resilient handling of failing token refresh (return still valid token)

- support both OAUTH2_ACCESS_TOKEN_URL and OAUTH_ACCESS_TOKEN_URL environment variables (OAUTH2_ACCESS_TOKEN_URL is used by Friboo)