bpo-43651: PEP 597: Fix EncodingWarning in some tests (GH-25142)

* test__xxsubinterpreters
* test_builtin
* test_doctest
* test_exceptions
* test_opcodes
* test_support
* test_argparse
* test_baseexception
* test_bdb
* test_bool
* test_asdl_parser
diff --git a/Lib/test/test_bool.py b/Lib/test/test_bool.py
index bec44d0..a3214c9 100644
--- a/Lib/test/test_bool.py
+++ b/Lib/test/test_bool.py
@@ -235,7 +235,7 @@ def test_boolean(self):
 
     def test_fileclosed(self):
         try:
-            with open(os_helper.TESTFN, "w") as f:
+            with open(os_helper.TESTFN, "w", encoding="utf-8") as f:
                 self.assertIs(f.closed, False)
             self.assertIs(f.closed, True)
         finally: