TEST: use marker nonfree for tests with Gurobi

And this can be used to mark tests with MOSEK and other
extra dependencies with restrictive licenses in the future.

These tests are not included by default and must be requested:

    pytest -m nonfree tests

tests/polytope_test.py

@@ -616,6 +616,7 @@ def test_reduce():
     assert_allclose(u, np.array([[50.], [1.]]), rtol=1e-07, atol=1e-07)
 
 
+@pytest.mark.nonfree
 def test_gurobipy_return_same_result_as_scipy():
     c, A, b = example_1d()
     result_gurobi = solvers.lpsolve(c, A, b, solver='gurobi')

tests/pytest.ini

@@ -0,0 +1,2 @@
+[pytest]
+addopts = -m 'not nonfree'