Skip tests that need boto3 if it's not installed

Two `mast` remote tests require `boto3`, which is not installed when
installing `astroquery` with the test dependencies. It therefore cannot
be assumed that `boto3` is installed when running the tests. If it isn't
installed then it is better to skip the tests in question than to have
them fail with `ModuleNotFoundError`.

Author: eerovaher

astroquery/mast/tests/test_mast_remote.py

@@ -386,6 +386,7 @@ def test_observations_download_file(self, tmp_path):
         assert result == ('COMPLETE', None, None)
 
     def test_get_cloud_uri(self):
+        pytest.importorskip("boto3")
         test_obs_id = '25568122'
 
         # get a product list
@@ -403,6 +404,7 @@ def test_get_cloud_uri(self):
         assert len(uri) > 0, f'Product for OBSID {test_obs_id} was not found in the cloud.'
 
     def test_get_cloud_uris(self):
+        pytest.importorskip("boto3")
         test_obs_id = '25568122'
 
         # get a product list