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_cmath.py b/Lib/test/test_cmath.py
index 668f27c..9bb9393 100644
--- a/Lib/test/test_cmath.py
+++ b/Lib/test/test_cmath.py
@@ -60,7 +60,7 @@ class CMathTests(unittest.TestCase):
     test_functions.append(lambda x : cmath.log(14.-27j, x))
 
     def setUp(self):
-        self.test_values = open(test_file)
+        self.test_values = open(test_file, encoding="utf-8")
 
     def tearDown(self):
         self.test_values.close()