bpo-40275: Remove test.support.TESTFN_ENCODING (GH-20482)

Replace test.support.TESTFN_ENCODING with sys.getfilesystemencoding().
diff --git a/Lib/test/test_sax.py b/Lib/test/test_sax.py
index ce3a422..bc77103 100644
--- a/Lib/test/test_sax.py
+++ b/Lib/test/test_sax.py
@@ -19,6 +19,7 @@
 import codecs
 import os.path
 import shutil
+import sys
 from urllib.error import URLError
 import urllib.request
 from test import support
@@ -35,7 +36,7 @@
 supports_nonascii_filenames = True
 if not os.path.supports_unicode_filenames:
     try:
-        support.TESTFN_UNICODE.encode(support.TESTFN_ENCODING)
+        support.TESTFN_UNICODE.encode(sys.getfilesystemencoding())
     except (UnicodeError, TypeError):
         # Either the file system encoding is None, or the file name
         # cannot be encoded in the file system encoding.