Drop variadic signature from Sopel memory types

[SnoopJ]

Requested Feature

Follow-up from #2525, see relevant comment on that issue.

SopelMemoryWithDefault and friends currently accept *args in their initializer signatures, but accept at most one positional argument. This issue is to keep track of our intent to replace the variadic form with the one-arg form.

This change should be done so that SopelIdentifierMemory's identifier_factory parameter must still be passed by keyword, using the keyword-only arguments syntax feature.

Problems Solved

No response

Alternatives

No response

Notes

This is not part of the 8.0.0 milestone since it is already long in the tooth, and #2525 is a pretty big set of changes there already.

[dgw]

Scrolling through the open issues, I noticed this and #2565 seem pretty closely related. I'm not sure they are similar enough to merge one into the other, though. For now I'll add a parent-child relationship.