Converting type_index to Python type

[lavaux]

The class pybind11::type allows the user to request a python type from a C++ type. However it does not allow to request from a type_index directly. I would like for example to have this kind of interface:

class type {
  // ...
  handle of(type_index const& index);
  ///...

In addition to the other of variants. I see from the internals that std::type_index is indeed used. So it would simple be a matter of exposing this to the user.