Retry more methods, like `delete_suppressions`

Hi,

I am using v1.25.1 and I received:

```
Postmark::TimeoutError

Net::ReadTimeout (Postmark::TimeoutError)
```

...error while using the `delete_suppressions` method.

I don't see a reason why it's not wrapped in `with_retries`, like many other calls.

Would you accept a PR that would add it to that and similar methods?                           