CI: change copt write test to use pytest skipif

[Transurgeon]

Description

Please include a short summary of the change.
This PR fixes some CI failures (not sure why they resurfaced now though) relating to skipping tests with unittest not working.
See the logs in the following recent build run:

cvxpy/tests/test_matrix_utilities.py::TestMatrixUtilities::test_convert
cvxpy/tests/test_matrix_utilities.py::TestMatrixUtilities::test_convert
cvxpy/tests/test_matrix_utilities.py::TestMatrixUtilities::test_convert
cvxpy/tests/test_matrix_utilities.py::TestMatrixUtilities::test_convert
cvxpy/tests/test_matrix_utilities.py::TestMatrixUtilities::test_convert
cvxpy/tests/test_matrix_utilities.py::TestMatrixUtilities::test_convert
  /home/runner/work/cvxpy/cvxpy/cvxpy/tests/test_matrix_utilities.py:56: PendingDeprecationWarning: the matrix subclass is not the recommended way to represent matrices or deal with linear algebra (see https://docs.scipy.org/doc/numpy/user/numpy-for-matlab-users.html). Please adjust your code to use regular ndarray.
    np.matrix(underlying_values, dtype=dtype),

cvxpy/tests/test_python_backends.py::TestSciPyBackend::test_tensor_view_add_dicts
  /home/runner/work/cvxpy/cvxpy/cvxpy/tests/test_python_backends.py:2538: SparseEfficiencyWarning: Comparing two sparse matrices using == is inefficient. Try using != instead.
    assert view.add_dicts({"a": one}, {"a": two}) == {"a": three}

cvxpy/tests/test_python_backends.py::TestSciPyBackend::test_tensor_view_add_dicts
  /home/runner/work/cvxpy/cvxpy/cvxpy/tests/test_python_backends.py:2540: SparseEfficiencyWarning: Comparing two sparse matrices using == is inefficient. Try using != instead.
    assert view.add_dicts({"a": {"c": one}}, {"a": {"c": one}}) == {"a": {"c": two}}

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
=========================== short test summary info ============================
FAILED cvxpy/tests/test_copt_write.py::test_write - unittest.case.SkipTest: COPT is not installed.
FAILED cvxpy/tests/test_gurobi_write.py::test_write - unittest.case.SkipTest: GUROBI is not installed.
========== 2 failed, 1280 passed, 447 skipped, 215 warnings in 38.33s ==========

Issue link (if applicable):

Type of change

• New feature (backwards compatible)
• New feature (breaking API changes)
• Bug fix
• Other (Documentation, CI, ...)

Contribution checklist

• Add our license to new files.
• Check that your code adheres to our coding style.
• Write unittests.
• Run the unittests and check that they’re passing.
• Run the benchmarks to make sure your change doesn’t introduce a regression.

[github-actions]

Benchmarks that have stayed the same:

   before           after         ratio
 [ab16d411]       [f9211a90]
      849±0ms          889±0ms     1.05  simple_QP_benchmarks.LeastSquares.time_compile_problem
     43.8±0ms         45.4±0ms     1.04  matrix_stuffing.SmallMatrixStuffing.time_compile_problem
      1.66±0s          1.72±0s     1.03  tv_inpainting.TvInpainting.time_compile_problem
      13.2±0s          13.6±0s     1.03  finance.CVaRBenchmark.time_compile_problem
      718±0ms          734±0ms     1.02  matrix_stuffing.ConeMatrixStuffingBench.time_compile_problem
      5.01±0s          5.12±0s     1.02  optimal_advertising.OptimalAdvertising.time_compile_problem
      343±0ms          349±0ms     1.02  gini_portfolio.Yitzhaki.time_compile_problem
      255±0ms          259±0ms     1.02  simple_QP_benchmarks.SimpleQPBenchmark.time_compile_problem
      1.09±0s          1.10±0s     1.01  finance.FactorCovarianceModel.time_compile_problem
      5.17±0s          5.24±0s     1.01  svm_l1_regularization.SVMWithL1Regularization.time_compile_problem
      242±0ms          245±0ms     1.01  high_dim_convex_plasticity.ConvexPlasticity.time_compile_problem
      22.7±0s          22.9±0s     1.01  sdp_segfault_1132_benchmark.SDPSegfault1132Benchmark.time_compile_problem
      239±0ms          241±0ms     1.01  gini_portfolio.Murray.time_compile_problem
      4.65±0s          4.69±0s     1.01  huber_regression.HuberRegression.time_compile_problem
      1.14±0s          1.15±0s     1.01  gini_portfolio.Cajas.time_compile_problem
      936±0ms          938±0ms     1.00  simple_LP_benchmarks.SimpleScalarParametrizedLPBenchmark.time_compile_problem
      1.87±0s          1.86±0s     1.00  simple_QP_benchmarks.UnconstrainedQP.time_compile_problem
      11.6±0s          11.4±0s     0.98  simple_LP_benchmarks.SimpleLPBenchmark.time_compile_problem
      2.37±0s          2.33±0s     0.98  simple_LP_benchmarks.SimpleFullyParametrizedLPBenchmark.time_compile_problem
      294±0ms          289±0ms     0.98  matrix_stuffing.ParamSmallMatrixStuffing.time_compile_problem
      3.29±0s          3.20±0s     0.97  quantum_hilbert_matrix.QuantumHilbertMatrix.time_compile_problem
      306±0ms          297±0ms     0.97  slow_pruning_1668_benchmark.SlowPruningBenchmark.time_compile_problem
      560±0ms          538±0ms     0.96  semidefinite_programming.SemidefiniteProgramming.time_compile_problem
      591±0ms          566±0ms     0.96  simple_QP_benchmarks.ParametrizedQPBenchmark.time_compile_problem
      1.48±0s          1.40±0s     0.94  matrix_stuffing.ParamConeMatrixStuffing.time_compile_problem