Generate an error if trying to use @Final with TypedDict (#7865)

Fixes #7849 

This fix modifies function `analyze_class` in `mypy/semanal.py`, if the analyzer identifies a `TypedDict`, it then traverses all decorators and checks if `@final` exists, and if so, generates an error.

mypy/semanal.py

@@ -1067,6 +1067,12 @@ def analyze_class(self, defn: ClassDef) -> None:
 
         is_typeddict, info = self.typed_dict_analyzer.analyze_typeddict_classdef(defn)
         if is_typeddict:
+            for decorator in defn.decorators:
+                decorator.accept(self)
+                if isinstance(decorator, RefExpr):
+                    if decorator.fullname in ('typing.final',
+                                              'typing_extensions.final'):
+                        self.fail("@final cannot be used with TypedDict", decorator)
             if info is None:
                 self.mark_incomplete(defn.name, defn)
             else:

test-data/unit/check-typeddict.test

@@ -1924,3 +1924,16 @@ v = {bad2: 2}  # E: Extra key 'bad' for TypedDict "Value"
 
 [builtins fixtures/dict.pyi]
 [typing fixtures/typing-full.pyi]
+
+[case testCannotUseFinalDecoratorWithTypedDict]
+from typing import TypedDict
+from typing_extensions import final
+
+@final  # E: @final cannot be used with TypedDict
+class DummyTypedDict(TypedDict):
+    int_val: int
+    float_val: float
+    str_val: str
+
+[builtins fixtures/dict.pyi]
+[typing fixtures/typing-full.pyi]