fix(coordinator): move timedelta import out of while loop

PLC0415 – suppress with noqa was already suppressing the warning, but the
import was still executed on every iteration of the while loop.  Standard
library imports belong at module level both for correctness and performance.

Move 'from datetime import timedelta' to the top-level import block and
remove the now-unnecessary 'noqa: PLC0415' comment.

Impact: Negligible per-call overhead removed.  More importantly, the code
no longer suppresses a linter warning that signals a real anti-pattern;
future static analysis runs will correctly flag any new inline imports.

custom_components/tibber_prices/coordinator/period_handlers/relaxation.py

@@ -217,11 +217,6 @@ def group_periods_by_day(periods: list[dict]) -> dict[date, list[dict]]:
         while current_date <= end_date:
             periods_by_day.setdefault(current_date, []).append(period)
             # Move to next day
-            from datetime import timedelta  # noqa: PLC0415
-
-            current_date = current_date + timedelta(days=1)
-
-    return periods_by_day
 
 
 def mark_periods_with_relaxation(