Introduce a type for an unknown invariant generic param that behaves strictly

This is the same issue as #1835, which was closed -- the given reason was that object can be used instead of Unknown. However, this is not the case. Consider:

class Foo(tp.Generic[T]):
  def bar(self, x: T) -> T:
    ...

  def baz(...):
    ...

I want a type that represents "Foo with unknown T", which I can call baz on but not bar.

foo: Foo[Unknown] = ...
foo.bar(1)  # type error, int cannot be assigned to Unknown
foo.baz()  # works fine because no T is needed

This doesn't work if I use object:

foo: Foo[object] = ...
foo.bar(1)  # typechecks because int is a subtype of object

この issue の例を確認し、#1835 での以前の議論と比較してください。不明なジェネリックパラメーターに対する望ましい動作を、どの呼び出しを受け入れ、どの呼び出しを拒否するかも含めて定義してください。型付けの動作と実装方針について合意できた時点で、この issue は完了です。