Dropped support for Python 3.6

Enabled annotations feature from future, which is only available from python 3.7 onwards.

Support for Python 3.9

• improve exception readability by printing the message when casting exception to string

SubclassIdentifiable custom exceptions

SubclassIdentifiable exceptions:

• raise UnhashableClassIdError instead of TypeError.
• raise InvalidClassIdError instead of ValueError.

Custom class registration exception and Docstring

• Raise UnidentifiableSubclassError when calling SubclassFactory constructor with an unidentifiable class.
• Added docstring with numpydoc style.

Factory custom exception

• Raise UnregisteredClassIdError when calling Factory.create with an unknown class id.

First Pypi release

• Factory:
  • Implemented class id. registration based Factory.
  • Implemented automatic subclass hierarchy registration Factory based on Identifiable-like class/interface.
• Observer:
  • Implemented publisher class.
  • Implemented event publisher class.
  • Implemented abstract subscriber class.
• Singleton:
  • Implemented SingletonMeta metaclass.
  • Implemented Singleton class.