Incompatibility with python3 in String validator.

[martin-re]

Python 3.10.12 (main, Nov 20 2023, 15:14:05) [GCC 11.4.0] on linux
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>>> 
>>> from pyArango.validation import String
>>> from pyArango.theExceptions import ValidationError
>>> 
>>> try:
...     String().validate(object)
... except ValidationError:
...     pass
... except Exception as e:
...     print(e)
... 
name 'unicode' is not defined

This issue is sub-problem of #215.