Swap deprecated urllib3 Retry arg 'method_whitelist' for 'allowed_methods'

[mgberg]

As of urllib 2.0.0 (released 2023-04-26), the method_whitelist argument for Retry (which has been deprecated for a while) was removed in favor of allowed_methods.

This PR simply swaps the old argument for the new one. The behavior should be unchanged. Closes #22.

More info on the allowed_methods argument can be found here.