Add pytest_markeval_namespace hook.

[bluetech]

This is same as #7695 but rebased, one tiny style issue and changed from testdir -> pytester.

Closes #7695

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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:

   def pytest_markeval_namespace():
       return {"color": "red"}

test_func.py:

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

[bluetech]

Merging per @nicoddemus's approval in #7695. Thanks @s0undt3ch!

[s0undt3ch]

Thanks!