tests: Use yaml.full_load when available.

Per <https://msg.pyyaml.org/load>.

tests/test_importers.py

@@ -14,9 +14,14 @@ from import2ledger import importers, strparse
 
 from . import DATA_DIR
 
+try:
+    load_yaml = yaml.full_load
+except AttributeError:
+    load_yaml = yaml.load
+
 class TestImporters:
     with pathlib.Path(DATA_DIR, 'imports.yml').open() as yaml_file:
-        test_data = yaml.load(yaml_file)
+        test_data = load_yaml(yaml_file)
     for test in test_data:
         test['source'] = DATA_DIR / test['source']