stubgen: do not include mypy generated symbols (python#18137)

Addresses part of python#18081

stubgen still does not handle dataclass transforms correctly but with
this change we make sure to never include private mypy generated symbols
in the stubs.

mypy/stubutil.py

@@ -832,6 +832,8 @@ def is_not_in_all(self, name: str) -> bool:
         return False
 
     def is_private_name(self, name: str, fullname: str | None = None) -> bool:
+        if "__mypy-" in name:
+            return True  # Never include mypy generated symbols
         if self._include_private:
             return False
         if fullname in self.EXTRA_EXPORTED:

test-data/unit/stubgen.test

@@ -4508,3 +4508,21 @@ class C3[T3 = int]: ...
 class C4[T4: int | float = int](list[T4]): ...
 
 def f5[T5 = int]() -> None: ...
+
+[case testIgnoreMypyGeneratedMethods_semanal]
+# flags: --include-private --python-version=3.13
+from typing_extensions import dataclass_transform
+
+# TODO: preserve dataclass_transform decorator
+@dataclass_transform()
+class DCMeta(type): ...
+class DC(metaclass=DCMeta):
+    x: str
+
+[out]
+class DCMeta(type): ...
+
+class DC(metaclass=DCMeta):
+    x: str
+    def __init__(self, x) -> None: ...
+    def __replace__(self, *, x) -> None: ...