Applies the three suggestions from #34513 review:
1. Drop the dead `isinstance(v, (int, float))` branch. psycopg2 and
psycopg3 always hand INTERVAL columns back as datetime.timedelta;
the numeric branch was unreachable in practice and was creating
false confidence in the tests covering it.
2. Move `_normalize_interval` from a `@staticmethod` on the class to a
module-level function. The previous form required
`INTERVAL: _normalize_interval.__func__ # type: ignore[attr-defined]`
to extract the underlying function from the staticmethod descriptor
inside the class body. Module-level lets `column_type_mutators`
reference the function directly — no `__func__`, no suppress.
3. Add a `logger.warning(...)` to the defensive return-None path so a
future driver surfacing something other than timedelta gets logged
rather than silently dropped.
Test updates: the numeric/bool assertions that exercised the removed
branch now exercise the defensive return-None path instead (same
expectations, but for the right reason).
b3604af4ce