bpo-31985: Deprecate openfp in aifc, sunau, and wave (#4344)

The openfp functions of aifp, sunau, and wave had pointed to the open
function of each module since 1993 as a matter of backwards
compatibility. In the case of aifc.openfp, it was both undocumented
and untested. This change begins the formal deprecation of those
openfp functions, with their removal coming in 3.9.

This additionally adds a TODO in test_pyclbr around using aifc.openfp,
though it shouldn't be changed until removal in 3.9.

Lib/test/audiotests.py

@@ -1,6 +1,7 @@
 from test.support import findfile, TESTFN, unlink
 import array
 import io
+from unittest import mock
 import pickle
 
 
@@ -49,6 +50,17 @@ class AudioTests:
             self.assertEqual(pickle.loads(dump), params)
 
 
+class AudioMiscTests(AudioTests):
+
+    def test_openfp_deprecated(self):
+        arg = "arg"
+        mode = "mode"
+        with mock.patch(f"{self.module.__name__}.open") as mock_open, \
+             self.assertWarns(DeprecationWarning):
+            self.module.openfp(arg, mode=mode)
+            mock_open.assert_called_with(arg, mode=mode)
+
+
 class AudioWriteTests(AudioTests):
 
     def create_file(self, testfile):