Retry more methods, like `delete_suppressions`

[gdubicki]

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?