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?

Beginne damit, die vorhandenen Nachrichtenkonstanten in mypy.messages und die Migration von Nachrichten aus mypy.checkexpr zu überprüfen. Untersuche anschließend die Fehlermeldungen in mypy.checkexpr, mypy.semanal, mypy.semanal_newtype und mypy.fastparse2. Die Aufgabe ist abgeschlossen, wenn die verbleibenden Fehlermeldungen in diesen Modulen Konstanten aus mypy.messages verwenden.