Use a syntax compatible with Python 3.8 for join context.

Author: jaraco

importlib_resources/tests/test_functional.py

@@ -1,6 +1,5 @@
 import unittest
 import os
-import contextlib
 
 from .compat.py39 import warnings_helper
 
@@ -171,21 +170,17 @@ def test_contents(self):
             set(c),
             {'utf-8.file', 'utf-16.file', 'binary.file', 'subdirectory'},
         )
-        with contextlib.ExitStack() as cm:
-            cm.enter_context(self.assertRaises(OSError))
-            cm.enter_context(
-                warnings_helper.check_warnings((".*contents.*", DeprecationWarning))
-            )
-
+        with self.assertRaises(OSError), warnings_helper.check_warnings((
+            ".*contents.*",
+            DeprecationWarning,
+        )):
             list(resources.contents(self.anchor01, 'utf-8.file'))
 
         for path_parts in self._gen_resourcetxt_path_parts():
-            with contextlib.ExitStack() as cm:
-                cm.enter_context(self.assertRaises(OSError))
-                cm.enter_context(
-                    warnings_helper.check_warnings((".*contents.*", DeprecationWarning))
-                )
-
+            with self.assertRaises(OSError), warnings_helper.check_warnings((
+                ".*contents.*",
+                DeprecationWarning,
+            )):
                 list(resources.contents(self.anchor01, *path_parts))
         with warnings_helper.check_warnings((".*contents.*", DeprecationWarning)):
             c = resources.contents(self.anchor01, 'subdirectory')