Fix: Wikipedia - save memory by replacing root.clear with elem.clear
The error you got is minor and appeared in the last version of pyarrow, we'll fix the CI to take this into account. You can ignore it
see: https://github.com/huggingface/datasets/issues/2031

What I did:
- replace root.clear with elem.clear
- remove lines to get root element
- $ make style
- $ make test
  - some tests required some pip packages, I installed them.

test results on origin/master and my branch are same. I think it's not related on my modification, isn't it?
```
==================================================================================== short test summary info ====================================================================================
FAILED tests/test_arrow_writer.py::TypedSequenceTest::test_catch_overflow - AssertionError: OverflowError not raised
============================================================= 1 failed, 2332 passed, 5138 skipped, 70 warnings in 91.75s (0:01:31) ==============================================================
make: *** [Makefile:19: test] Error 1

```

Is there anything else I should do?