Migrate remaining failure messages to mypy.messages

I'm working on a tool to provide a more gradual mypy adoption path, by providing a fine grained configuration for selecting or ignoring mypy errors. My current prototype uses regular expressions, but maintaining a comprehensive set of regular expressions is both time consuming and brittle, as existing messages could change, and new ones will be added.

I noticed that an effort has already been made to move error messages from mypy.checkexpr into mypy.messages. Making this set of constants complete would solve my problem, and hopefully be of benefit to you all as well. Would you accept a pull request to migrate the remaining failure messages from checkexpr, semanal, semanal_newtype, and fastparse2 to messages constants?

Empieza revisando las constantes de mensajes existentes en mypy.messages y cómo se migraron los mensajes desde mypy.checkexpr. Después, inspecciona los mensajes de error en mypy.checkexpr, mypy.semanal, mypy.semanal_newtype y mypy.fastparse2. El trabajo estará terminado cuando los mensajes de error restantes en esos módulos usen constantes de mypy.messages.