Use typing.List for compatibility with Python 3.6

conservancy_beancount/reconcile/statement_reconciler.py

@@ -256,7 +256,7 @@ def format_record(record: dict) -> str:
     return output
 
 
-def format_multirecord(r1s: list[dict], r2s: list[dict], note: str) -> list[list]:
+def format_multirecord(r1s: List[dict], r2s: List[dict], note: str) -> List[list]:
     """Generates output lines for one statement:multiple books transaction match."""
     assert len(r1s) == 1
     assert len(r2s) > 1