[kroplabeskidu]

Description

Item 11 from #6159. Calling pybind11_object() from Python, or a Python subclass of it with no registered C++ base, aborts the process because a C++ exception reaches the interpreter through the extern "C" tp_new:

terminate called after throwing an instance of 'std::runtime_error'
  what():  instance allocation failed: new instance has no pybind11-registered base types

• pybind11_object_new now turns exceptions into Python errors, so this case raises TypeError.
• make_new_instance checks the tp_alloc result, and releases the object if allocate_layout() throws.
• clear_instance now uses the values_and_holders(PyObject *) constructor, which gives an empty iteration for a type with no registered base. Without this, releasing the object segfaulted in a release build, and the iterator has an assert for this case in debug builds.

The new test aborts the test process without this fix. I ran it in both release and debug builds.

Suggested changelog entry:

• Instantiating pybind11_object from Python now raises TypeError instead of aborting the process.