stdlib/2and3/builtins: change dict.fromkeys to classmethod

[bluetech]

The referenced issue python/mypy#328 is fixed.

[JelleZijlstra]

What difference does this actually make? dict.fromkeys is a builtin method and isn't an instance of either the classmethod or staticmethod type.

The only observable difference I can think of is that classmethods can return an instance of a subclass if called on a subclass, and that's indeed what happens if you call fromkeys, but the current stubs don't express that behavior anyway.

The docs (https://docs.python.org/3.8/library/stdtypes.html#dict.fromkeys) do call the method a classmethod, so I'm happy to merge this PR as is, but I'm just curious whether this solves some concrete problem.

[bluetech]

@JelleZijlstra This indeed started in an attempt to make the cls generic (#3800).

Since we won't be going this route, I can change this PR to just drop the TODO note instead. Though it does seem better to keep the door open for generic self, so I do slightly prefer to merge as-is.

[JelleZijlstra]

Sounds good, I merged it.