Use older 3.9 union syntax.

Author: ionelmc

src/pytest_cov/engine.py

@@ -12,6 +12,7 @@
 import warnings
 from io import StringIO
 from pathlib import Path
+from typing import Union
 
 import coverage
 from coverage.data import CoverageData
@@ -68,9 +69,7 @@ def _data_suffix(name):
 class CovController:
     """Base class for different plugin implementations."""
 
-    cov: coverage.Coverage | None
-
-    def __init__(self, options: argparse.Namespace, config: None | object, nodeid: None | str):
+    def __init__(self, options: argparse.Namespace, config: Union[None, object], nodeid: Union[None, str]):
         """Get some common config used by multiple derived classes."""
         self.cov_source = options.cov_source
         self.cov_report = options.cov_report