Fix bad type hint on test variable

The type hint should be `Schema` and not a `MutableMapping`.

tests/test_tap.py

@@ -23,8 +23,6 @@
 import shutil
 import tempfile
 import unittest
-from collections.abc import MutableMapping
-from typing import Any
 
 import sqlalchemy
 import yaml
@@ -39,7 +37,7 @@
 class VisitorTestCase(unittest.TestCase):
     """Test the TAP loading visitor."""
 
-    schema_obj: MutableMapping[str, Any] = {}
+    schema_obj: Schema
 
     def setUp(self) -> None:
         """Load data from a test file."""