We can move this into tp_dealloc now.
Only call unset() in the slice allocate case since g_boxed_free() will
do it anyway.

This allows us to assume that the wrapper is always valid and remove
the _is_valid attribute.

One difference now is that g_boxed_free() is called when the wrapper can no longer
be used and that breaks the custom glib sources where g_boxed_free() triggers
the finalize callback which tries to call into a method of the wrapper.

To work around this provide a _clear_boxed() method which allows the wrapper
to free the underlying boxed in __del__ where the wrapper is still usable.

After chaining up accessing anything on the instance will crash like
accessing self.g_type in this case. Guard against that by exposing if the
wrapped boxed is still there.

Ideally we shouldn't invalidate the object in __del__, but various
things depend on it atm. For another time..

TypeError is wrong there anyway.

No need to have two similarly named files

This asserts that PyType_Ready() was called on the types before using them
to create instances or as base class for other types.

With this PyPy no longer crashes when importing the _gi module.