Fixed timezone being returned incorrectly

Happened if you've set TZ to non-UTC.

Apparently astimezone and replace doesn't do the same thing even if tzinfo is None.

spoolman/api/v1/models.py

@@ -13,7 +13,7 @@ from spoolman.math import length_from_weight
 def datetime_to_str(dt: datetime) -> str:
     """Convert a datetime object to a string."""
     if dt.tzinfo is None:
-        dt = dt.astimezone(tz=timezone.utc)
+        dt = dt.replace(tzinfo=timezone.utc)
     return dt.isoformat().replace("+00:00", "Z")