bpo-43651: PEP 597: Fix EncodingWarning in some tests (GH-25145)

* test_asyncio
* test_bz2
* test_math
* test_cmath
* test_cmd_line
* test_cmd_line_script
* test_compile
* test_contextlib
* test_profile
* ctypes/test/test_find
* test_multiprocessing
* test_configparser
* test_csv
* test_dbm_dumb
* test_decimal
* test_difflib
* os.fdopen() calls io.text_encoding() to emit EncodingWarning for right place.
diff --git a/Lib/test/test_contextlib.py b/Lib/test/test_contextlib.py
index 50943c1..453ef6c 100644
--- a/Lib/test/test_contextlib.py
+++ b/Lib/test/test_contextlib.py
@@ -316,13 +316,13 @@ def testWithOpen(self):
         tfn = tempfile.mktemp()
         try:
             f = None
-            with open(tfn, "w") as f:
+            with open(tfn, "w", encoding="utf-8") as f:
                 self.assertFalse(f.closed)
                 f.write("Booh\n")
             self.assertTrue(f.closed)
             f = None
             with self.assertRaises(ZeroDivisionError):
-                with open(tfn, "r") as f:
+                with open(tfn, "r", encoding="utf-8") as f:
                     self.assertFalse(f.closed)
                     self.assertEqual(f.read(), "Booh\n")
                     1 / 0