Adapt to python3.8 C API change

Do `Py_DECREF(type)` on all python objects on deallocation

fix pybind#1946

Author: sizmailov

include/pybind11/detail/class.h

@@ -350,13 +350,19 @@ extern "C" inline void pybind11_object_dealloc(PyObject *self) {
     auto type = Py_TYPE(self);
     type->tp_free(self);
 
+#if PY_VERSION_HEX < 0x03080000
     // `type->tp_dealloc != pybind11_object_dealloc` means that we're being called
     // as part of a derived type's dealloc, in which case we're not allowed to decref
     // the type here. For cross-module compatibility, we shouldn't compare directly
     // with `pybind11_object_dealloc`, but with the common one stashed in internals.
     auto pybind11_object_type = (PyTypeObject *) get_internals().instance_base;
     if (type->tp_dealloc == pybind11_object_type->tp_dealloc)
         Py_DECREF(type);
+#else
+    // This was not needed before Python 3.8 (Python issue 35810)
+    // https://github.com/pybind/pybind11/issues/1946
+    Py_DECREF(type);
+#endif
 }
 
 /** Create the type which can be used as a common base for all classes.  This is