[CI] remove unused import in python tests

[jameslamb]

👋 I'm interested in spending some time helping out on xgboost, so I started looking around tonight.

I ran flake8 over the Python package tonight, and found what looks like an unused import.

flake8 \
    --ignore=E501,W503 \
    python-package/xgboost

python-package/xgboost/testing/__init__.py:6:1: F401 'copy' imported but unused

This PR proposes removing it.

How I tested this

locally on my mac (click for details)

Basically copied what I saw in

xgboost/.github/workflows/python_tests.yml

Line 157 in 0db903b

- name: Build XGBoost on macos

.

Created and activated a conda environment.

mamba env create \
    --name xgboost-dev \
        pythoon=3.10 \
        pandas \
        numpy \
        scipy \
        dask \
        distributed \
        hypothesis \
        pytest \
        pytest-cov \
        matplotlib

source activate xgboost-dev

Then compiled, built the Python package, installed it, and ran the Python tests.

brew install ninja
mkdir build
cd build
cmake \
    -GNinja \
    -DGOOGLE_TEST=ON \
    -DUSE_DMLC_GTEST=ON \
    -DCMAKE_PREFIX_PATH=$CONDA_PREFIX \
    ..

ninja -j2

cd ../python-package
python setup.py install

cd ../
pytest tests/python

This resulted in 3 test failures which I'm guessing are not related to the change in this PR.

FAILED tests/python/test_linear.py::TestLinear::test_coordinate - hypothesis.errors.FailedHealthCheck: Examples routinely exceeded the max allowable size...
FAILED tests/python/test_tracker.py::test_rabit_ops - socket.gaierror: [Errno 8] nodename nor servname provided, or not known
FAILED tests/python/test_tracker.py::test_rank_assignment - socket.gaierror: [Errno 8] nodename nor servname provided, or not known

Notes for Reviewers

Thanks very much for your time and consideration.