text: Title: [BUG] `sktime` fails if an older version of `polars` is installed
  Body: Reported by @wirrywoo on discord.
  
  If an older version of `polars` is installed, `sktime` fails due to import chains and module level generation of a test fixture with `DataFrame(strict=False)`, where the `strict` argument is not present in earlier `polars` versions.
  
  The solution is to add the fixture only on `polars` versions that have the argument, and a workaround is to avoid older `polars` versions.