Add TODO comments

Author: seisman

pygmt/clib/session.py

@@ -1883,6 +1883,7 @@ def virtualfile_in(
                 _data = [x, y]
                 if z is not None:
                     _data.append(z)
+                # TODO(PyGMT>=0.20.0): Remove the deprecated parameter 'extra_arrays'.
                 if extra_arrays:
                     msg = (
                         "The parameter 'extra_arrays' will be removed in v0.20.0. "

pygmt/tests/test_clib_virtualfile_in.py

@@ -130,6 +130,7 @@ def test_virtualfile_in_matrix_string_dtype():
                 # not lib.virtualfile_from_matrix, but it's technically complicated.
 
 
+# TODO(PyGMT>=0.20.0): Remove the test related to deprecated parameter 'extra_arrays'.
 def test_virtualfile_in_extra_arrays(data):
     """
     Test that the extra_arrays parameter is deprecated.