Suggest PEP 604 annotation when on Python 3.10

Code running on Python 3.10 can use PEP 604 annotations, so suggest
those instead.

mypy/messages.py

@@ -1193,14 +1193,18 @@ def need_annotation_for_var(self, node: SymbolNode, context: Context,
                                 python_version: Optional[Tuple[int, int]] = None) -> None:
         hint = ''
         has_variable_annotations = not python_version or python_version >= (3, 6)
+        pep604_supported = python_version and python_version >= (3, 10)
         # type to recommend the user adds
         recommended_type = None
         # Only gives hint if it's a variable declaration and the partial type is a builtin type
         if python_version and isinstance(node, Var) and isinstance(node.type, PartialType):
             type_dec = '<type>'
             if not node.type.type:
                 # partial None
-                recommended_type = f'Optional[{type_dec}]'
+                if pep604_supported:
+                    recommended_type = f'{type_dec} | None'
+                else:
+                    recommended_type = f'Optional[{type_dec}]'
             elif node.type.type.fullname in reverse_builtin_aliases:
                 # partial types other than partial None
                 alias = reverse_builtin_aliases[node.type.type.fullname]

test-data/unit/check-inference.test

@@ -3255,3 +3255,7 @@ reveal_type(x)  # N: Revealed type is "builtins.bytes"
 if x:
     reveal_type(x)  # N: Revealed type is "builtins.bytes"
 [builtins fixtures/dict.pyi]
+
+[case testSuggestPep604AnnotationForPartialNone]
+# flags: --local-partial-types --python-version 3.10
+x = None # E: Need type annotation for "x" (hint: "x: <type> | None = ...")