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_baseexception.py b/Lib/test/test_baseexception.py
index c324682..8db497a 100644
--- a/Lib/test/test_baseexception.py
+++ b/Lib/test/test_baseexception.py
@@ -28,8 +28,9 @@ def test_inheritance(self):
             except TypeError:
                 pass
 
-        inheritance_tree = open(os.path.join(os.path.split(__file__)[0],
-                                                'exception_hierarchy.txt'))
+        inheritance_tree = open(
+                os.path.join(os.path.split(__file__)[0], 'exception_hierarchy.txt'),
+                encoding="utf-8")
         try:
             superclass_name = inheritance_tree.readline().rstrip()
             try: