I had not added tests for issue25961 because couldn't find a good place for adding new tests. It looks that there are no special tests for creating new type with type() at all.

Proposed patch adds separate class with tests in test_builtin (may be this is not the best place, I'm not sure). They test type() with valid and invalid arguments, and also test setting some attributes, because error handling should be consistent for constructor and setters. There are some of setting tests in test_descr, but this doesn't look a good place for testing the constructor.