Make ErrorType a proper Enum

[cdce8p]

Make ErrorType inherit from Enum. With this change mypy will infer the following

from av.error import ErrorType

reveal_type(ErrorType)                # def (value: object) -> av.error.ErrorType
reveal_type(ErrorType.BSF_NOT_FOUND)  # Literal[ErrorType.BSF_NOT_FOUND]
reveal_type(ErrorType.BSF_NOT_FOUND.name)   # str
reveal_type(ErrorType.BSF_NOT_FOUND.value)  # int
reveal_type(ErrorType.BSF_NOT_FOUND.__reduce__())  # tuple[Callable[[str, str, str], EnumItem], tuple[str, str, str]]