do not specialize private members

it's unsafe to specialize private members:
```py
class A[T]: # covariant
def __init__(self, t: T):
self._t = t

def get(self) -> T:
return self._t

def f(self):
a = A(1)
b: A[object] = a
b._t = "what?"
a._t + 1 # runtime error
```

this can also manifest with annotated `self`:
```py
class A[T]:
def __init__(self, t: T):
self._t = t

def get(self) -> T:
return self._t

def set(self: A[object], value: str):
self._t = value # expect error

a = A[int](1)
a.set("a string???, i hope it doesn't set `_t`")
a.get() + 1
```

this also applies to contravariant type parameters and private methods

if we instead never specialize private members, then we can only interact them safely with generic values, preventing this unsoundness