- Replaced section heading with "The type of class objects" and
  changed the opening sentence to match.

- Drop "at runtime".

- Replaced "is not allowed" with "is rejected by the type checker"
  (and then some).

- Clarified that you can write Type[Union[...]] or Type[Any].

- Mentioned that a future PEP revision may deal with constructor
  signatures.

- Clarified behavior when Type[Any] is used.

- Clarify that Type[T] is covariant (though I'm not sure I understand
  the concept well enough here to explain it well -- by contrast, if
  Type[T] were invariant, what would change?).