B306 should ignore cases where BaseException subclass defines message property

[li-wjohnson]

Currently, every caught exception with a .message is flagged, even if the class defines self.message explicitly.

[Julian]

I am using bugbear for the first time like 3 seconds ago, so feel free to ignore me :) but it seems like maybe it's worth removing this check entirely given Py3-land now.

It seems historically more relevant of a prep warning for Py2 users, but by now, if someone's using .message, it's a better static assumption that it's been explicitly defined than that the code will blow up at runtime, no?