Vetoes

This release remains compatible with rejected versions back to 3.12 at runtime but the tests will only run on rejected 3.18 and newer.

Contains a few useful mixins at this point:
- `vetoes.service.HTTPServiceMixin`: makes HTTP API calls a little easier
- `vetoes.config.FeatureFlagMixin`: adds the ability to configure `self.feature_flags['name']`
- `vetoes.config.TimeoutConfigurationMixin`: adds the ability to configure timeouts and retrieve them with `self.get_timeout('name')`