Move pytest import inside of skip_if_no function (#2891)

To fix the `ModuleNotFoundError: No module named 'pytest'` error
in the cache_data.yaml GitHub Action.

pygmt/helpers/testing.py

@@ -6,7 +6,6 @@
 import os
 import string
 
-import pytest
 from pygmt.exceptions import GMTImageComparisonFailure
 from pygmt.io import load_dataarray
 from pygmt.src import which
@@ -272,6 +271,8 @@ def skip_if_no(package):
         A pytest.mark.skipif to use as either a test decorator or a
         parametrization mark.
     """
+    import pytest
+
     try:
         _ = importlib.import_module(name=package)
         has_package = True