Mark xarray shading test fail for GMT<=6.1.1 (#649)

The xarray shading issue in #364 was fixed by upstream GMT in GenericMappingTools/gmt#4328.

This PR updates the pytest xfail condition so that the xarray shading test is expected to xfail only for GMT<=6.1.1.

pygmt/tests/test_grdimage.py

@@ -4,12 +4,16 @@
 import numpy as np
 import pytest
 import xarray as xr
+from packaging.version import Version
 
-from .. import Figure
+from .. import Figure, clib
 from ..datasets import load_earth_relief
 from ..exceptions import GMTInvalidInput
 from ..helpers.testing import check_figures_equal
 
+with clib.Session() as _lib:
+    gmt_version = Version(_lib.info["version"])
+
 
 @pytest.fixture(scope="module", name="grid")
 def fixture_grid():
@@ -69,7 +73,10 @@ def test_grdimage_file():
     return fig
 
 
-@pytest.mark.xfail(reason="Upstream bug in GMT 6.1.1")
+@pytest.mark.xfail(
+    reason="Upstream bug in GMT 6.1.1",
+    condition=gmt_version <= Version("6.1.1"),
+)
 @check_figures_equal()
 def test_grdimage_xarray_shading(grid):
     """