Add a new hook , `pytest_markeval_namespace` which should return a dictionary.
This dictionary will be used to augment the "global" variables available to evaluate skipif/xfail/xpass markers.

Pseudo example

``conftest.py``:

.. code-block:: python
   def pytest_markeval_namespace():
       return {"color": "red"}
``test_func.py``:

.. code-block:: python
   @pytest.mark.skipif("color == 'blue'", reason="Color is not red")
   def test_func():
       assert False