Code modernizations (Python 3.8)

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Currently the minimum required version of Python is 3.7, which lacks some features that would improve the code a bit.

Things to improve when raising the minimum required version of Python to 3.8 or above:

• validataclasses: Assigning tuples without parentheses to fields with type annotations (correct examples and unit tests),
  e.g. foo: int = (IntegerValidator(), Default(0)) to foo: int = IntegerValidator(), Default(0)
• @validataclass decorator: positional-arguments-only operator: def validataclass(cls=None, /, **kwargs):