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_doctest.py b/Lib/test/test_doctest.py
index 6a5013f..6f51b1b 100644
--- a/Lib/test/test_doctest.py
+++ b/Lib/test/test_doctest.py
@@ -2846,7 +2846,7 @@ def test_CLI(): r"""
     >>> from test.support.os_helper import temp_dir
     >>> with temp_dir() as tmpdir:
     ...     fn = os.path.join(tmpdir, 'myfile.doc')
-    ...     with open(fn, 'w') as f:
+    ...     with open(fn, 'w', encoding='utf-8') as f:
     ...         _ = f.write('This is a very simple test file.\n')
     ...         _ = f.write('   >>> 1 + 1\n')
     ...         _ = f.write('   2\n')
@@ -2898,7 +2898,7 @@ def test_CLI(): r"""
     >>> from test.support.os_helper import temp_dir
     >>> with temp_dir() as tmpdir:
     ...     fn = os.path.join(tmpdir, 'myfile.doc')
-    ...     with open(fn, 'w') as f:
+    ...     with open(fn, 'w', encoding="utf-8") as f:
     ...         _ = f.write('This is another simple test file.\n')
     ...         _ = f.write('   >>> 1 + 1\n')
     ...         _ = f.write('   2\n')
@@ -2909,7 +2909,7 @@ def test_CLI(): r"""
     ...         _ = f.write('\n')
     ...         _ = f.write('And that is it.\n')
     ...     fn2 = os.path.join(tmpdir, 'myfile2.py')
-    ...     with open(fn2, 'w') as f:
+    ...     with open(fn2, 'w', encoding='utf-8') as f:
     ...         _ = f.write('def test_func():\n')
     ...         _ = f.write('   \"\"\"\n')
     ...         _ = f.write('   This is simple python test function.\n')