Fix pylint style errors

.pylintrc

@@ -441,6 +441,7 @@ function-naming-style=snake_case
 good-names=i,
            j,
            k,
+           df,
            ex,
            Run,
            _,

pygmt/tests/test_x2sys_cross.py

@@ -6,20 +6,23 @@
 
 import pandas as pd
 import pytest
-import xarray as xr
 
 from .. import which, x2sys_cross, x2sys_init
-from ..datasets import load_sample_bathymetry
-from ..exceptions import GMTInvalidInput
-from ..helpers import data_kind
 
+# from ..datasets import load_sample_bathymetry
+# from ..exceptions import GMTInvalidInput
 
-@pytest.fixture
-def mock_x2sys_home(monkeypatch):
+
+@pytest.fixture(name="mock_x2sys_home")
+def fixture_mock_x2sys_home(monkeypatch):
+    """
+    Set the X2SYS_HOME environment variable to the current working directory
+    for the test session
+    """
     monkeypatch.setenv("X2SYS_HOME", os.getcwd())
 
 
-def test_x2sys_cross_input_file(mock_x2sys_home):
+def test_x2sys_cross_input_file(mock_x2sys_home):  # pylint: disable=unused-argument
     """
     Run x2sys_cross by passing in a filename
     """

pygmt/x2sys.py

@@ -2,7 +2,6 @@
 GMT supplementary X2SYS module for crossover analysis.
 """
 import pandas as pd
-import xarray as xr
 
 from .clib import Session
 from .helpers import (
@@ -13,7 +12,8 @@
     data_kind,
     dummy_context,
 )
-from .exceptions import GMTInvalidInput
+
+# from .exceptions import GMTInvalidInput
 
 
 @fmt_docstring