Make NoneType annotation error a new code

Change the code of "NoneType should not be used as a type, please use None instead" error to be a new code, so that people who don't like this rule can disable it easily.

I haven't made this a subcode of valid-type because NoneType is actually a valid type; it's just dispreferred.

I have added the documentation for the new code, but haven't added any new tests (the old test still runs fine).

Fixes #20218

Author: wyattscarpenter

docs/source/error_code_list.rst

@@ -215,6 +215,27 @@ You can use :py:class:`~collections.abc.Callable` as the type for callable objec
         for x in objs:
             f(x)
 
+.. _code-nonetype-type:
+
+Check that NoneType is not used as a type (annotation) [nonetype-type]
+----------------------------------------------------------------------
+
+The preferred way to annotate the type of `None` is `None`.
+`NoneType` is equivalent, but mypy won't allow it by default.
+
+.. code-block:: python
+    from types import NoneType
+    def f(x: None) -> None:
+        reveal_type(x) # note: Revealed type is "None"
+
+    # error: NoneType should not be used as a type, please use None instead  [nonetype-type]
+    def g(x: NoneType) -> None:
+        reveal_type(x) # note: Revealed type is "None"
+
+    # error: NoneType should not be used as a type, please use None instead  [nonetype-type]
+    x1: NoneType = None
+    x2: None = None #OK
+
 .. _code-metaclass:
 
 Check the validity of a class's metaclass [metaclass]

mypy/errorcodes.py

@@ -60,6 +60,7 @@ def __hash__(self) -> int:
     "call-overload", "Check that an overload variant matches arguments", "General"
 )
 VALID_TYPE: Final = ErrorCode("valid-type", "Check that type (annotation) is valid", "General")
+NONETYPE_TYPE: Final = ErrorCode("nonetype-type", "Check that type (annotation) is not NoneType", "General")
 VAR_ANNOTATED: Final = ErrorCode(
     "var-annotated", "Require variable annotation if type can't be inferred", "General"
 )

mypy/typeanal.py

@@ -920,7 +920,7 @@ def analyze_type_with_type_info(
             self.fail(
                 "NoneType should not be used as a type, please use None instead",
                 ctx,
-                code=codes.VALID_TYPE,
+                code=codes.NONETYPE_TYPE,
             )
             return NoneType(ctx.line, ctx.column)