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

检查此 issue 中的示例，并将其与 #1835 中之前的讨论进行比较。定义未知泛型参数的预期行为，包括接受或拒绝哪些调用；当类型检查行为和实现路径达成一致时，该 issue 即告完成。