Avoid accessing protected class members from outside

[textbook]

Per the QuantifiedCode report, there is currently one issue, in the implementation of aslack.utils.raise_for_status. This is potentially upstream in aiohttp, as it seems useful to check whether it's a move-based exception before adding the location header to it, but consider other, public ways of approaching it first.

[textbook]

This required a pylint disable flag (see 693db6d) which isn't ideal.