[#14] `AutoRegister` now returns base class instead of metaclass

[todofixthis]

closes #14

⚠️ Deprecations

• class_registry.auto_register.AutoRegister is deprecated and will be removed in a future version of ClassRegistry. Use class_registry.base.AutoRegister instead, and note that the latter returns a base class instead of a metaclass. You'll need to update your classes like this:

  # Deprecated:
  from class_registry.auto_register import AutoRegister
  class MyBaseClass(metaclass=AutoRegister(my_registry)):
      ...
  
  # Update to this:
  from abc import ABC
  from class_registry.base import AutoRegister
  class MyBaseClass(AutoRegister(my_registry), ABC):
      ...

Minor changes

• Cleared out heaps of obsolete backwards-compatibility logic.