Make code compatible with Python 3.8

Author: Sergey Vasilyev

data_diff/diff_tables.py

@@ -6,7 +6,7 @@
 from enum import Enum
 from contextlib import contextmanager
 from operator import methodcaller
-from typing import Dict, Set, List, Tuple, Iterator, Optional
+from typing import Dict, Set, List, Tuple, Iterator, Optional, Union
 from concurrent.futures import ThreadPoolExecutor, as_completed
 
 import attrs
@@ -262,7 +262,7 @@ def _validate_and_adjust_columns(self, table1: TableSegment, table2: TableSegmen
 
  def _diff_tables_root(
  self, table1: TableSegment, table2: TableSegment, info_tree: InfoTree
- ) -> DiffResult | DiffResultList:
+ ) -> Union[DiffResult, DiffResultList]:
  return self._bisect_and_diff_tables(table1, table2, info_tree)
 
  @abstractmethod