Modify `CTAToolInterface`'s method types

[AleksBekker]

There are two main changes that need to occur:

1. Make the requirements getter an abstractmethod and remove its setter.
   Currently, the programmer needs to set the requirements using the setter.
   However, the requirements property should not be modifyable after the class
   is created. Making the requirements getter abstract and removing its setter
   would remove this modifability while also aligning better with the typical
   "abstract class."

2. Make all of the CTA-related methods static. These methods never access any
   attributes or properties, so they should be made static.