remind: fix import streamlining gone haywire

[dgw]

"Unexpected AttributeError: 'str' object has no attribute 'utc'" begone.

Fixes a bug I introduced in #2468 by not being careful enough with my imports. Code review or not, I should've caught it. (And I was reminded to finish/open this PR by mypy --check-untyped-defs flagging these errors, so that's all the more reason to work toward #2461 in the longer term.)

Checklist

• [x] I have read CONTRIBUTING.md
• [x] I can and do license this contribution under the EFLv2
• [x] No issues are reported by make qa (runs make quality and make test)
• [x] I have tested the functionality of the things this change touches

Notes

mypy --check-untyped-defs still reports errors in sopel/modules/remind.py after this patch, but they'll be fixed separately. @SnoopJ and I have been chipping away at that list over the weekend.

[dgw]

this would also be a good opportunity to add a test that would have caught the bug

Went for the minimum-necessary-change approach because:

1. As discovered shortly after, mypy --check-untyped-defs (the new default behavior of make mypy, soon) flags this problem
2. More importantly, the not-so-long-term plan is to replace this built-in plugin with an external sopel-remind package that already exists and has much better test coverage

Basically, I was lazy in favor of just getting it working again so our master-based bot unbreaks. Once the new package has optional-date support for .at, we can just drop the builtin.