pythongh-91896: Deprecate collections.abc.ByteString

Getting a DeprecationWarning on issubclass proved to be difficult,
because it could affect unrelated looking things like
`isinstance(bytes, Sequence)`

Lib/_collections_abc.py

@@ -1064,8 +1064,27 @@ def count(self, value):
 Sequence.register(range)
 Sequence.register(memoryview)
 
-
-class ByteString(Sequence):
+class _DeprecateByteStringMeta(ABCMeta):
+    def __new__(cls, name, bases, namespace, **kwargs):
+        if name != "ByteString":
+            import warnings
+
+            warnings._deprecated(
+                "collections.abc.ByteString",
+                remove=(3, 14),
+            )
+        return super().__new__(cls, name, bases, namespace, **kwargs)
+
+    def __instancecheck__(cls, instance):
+        import warnings
+
+        warnings._deprecated(
+            "collections.abc.ByteString",
+            remove=(3, 14),
+        )
+        return super().__instancecheck__(instance)
+
+class ByteString(Sequence, metaclass=_DeprecateByteStringMeta):
     """This unifies bytes and bytearray.
 
     XXX Should add all their methods.
@@ -1076,7 +1095,6 @@ class ByteString(Sequence):
 ByteString.register(bytes)
 ByteString.register(bytearray)
 
-
 class MutableSequence(Sequence):
     """All the operations on a read-write sequence.
 

Lib/test/test_collections.py

@@ -1940,14 +1940,21 @@ def assert_index_same(seq1, seq2, index_args):
 
     def test_ByteString(self):
         for sample in [bytes, bytearray]:
-            self.assertIsInstance(sample(), ByteString)
+            with self.assertWarns(DeprecationWarning):
+                self.assertIsInstance(sample(), ByteString)
             self.assertTrue(issubclass(sample, ByteString))
         for sample in [str, list, tuple]:
-            self.assertNotIsInstance(sample(), ByteString)
+            with self.assertWarns(DeprecationWarning):
+                self.assertNotIsInstance(sample(), ByteString)
             self.assertFalse(issubclass(sample, ByteString))
-        self.assertNotIsInstance(memoryview(b""), ByteString)
+        with self.assertWarns(DeprecationWarning):
+            self.assertNotIsInstance(memoryview(b""), ByteString)
         self.assertFalse(issubclass(memoryview, ByteString))
-        self.validate_abstract_methods(ByteString, '__getitem__', '__len__')
+        with self.assertWarns(DeprecationWarning):
+            self.validate_abstract_methods(ByteString, '__getitem__', '__len__')
+
+        with self.assertWarns(DeprecationWarning):
+            class X(ByteString): pass
 
     def test_MutableSequence(self):
         for sample in [tuple, str, bytes]: