Detect methods that hardcode class names in their return type, but shouldn't

[AlexWaygood]

Currently, Y019 is quite conservative: it only suggests using _typeshed.Self if the method is already using a TypeVar. But there are other common mistakes that we could easily look for in another error code, many of which were highlighted by python/typeshed#7070, where users will often hardcode class names in the return type, but they should really be using a TypeVar:

• __new__ methods.
• __(a)iter__ methods if the class inherits from typing.Iterator or collections.abc.Iterator.
• __(a)enter__ methods.
• Maybe classmethods that have names starting with from_ (e.g. int.from_bytes, str.from_hex, etc)
• Any others?

Some of these might be noisier than others, but I'd like to try it out and see what the signal-to-noise ratio is like.