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

Parser/asdl.py

@@ -204,7 +204,7 @@ def check(mod):
 
 def parse(filename):
     """Parse ASDL from the given file and return a Module node describing it."""
-    with open(filename) as f:
+    with open(filename, encoding="utf-8") as f:
         parser = ASDLParser()
         return parser.parse(f.read())