`pyrefly report`: ignore trivial return types of special methods and attributes

[jorenham]

For example, the return type of __init__ is always None, so annotated or unannotated, it makes no difference for users. And although some type-checkers require it, I've also seen a couple of libraries that never annotate the return type of __init__.

See jorenham/typestats#353 for the full list that typestats uses.