Fix style of type comparisons in some unit tests

[flauschzelle]

While I was setting up the dev environment, I ran the tests as described in the readme, and got some error messages from flake8:

tests/validators/allow_empty_string_test.py:33:16: E721 do not compare types, for exact checks use `is` / `is not`, for instance checks use `isinstance()`
tests/validators/allow_empty_string_test.py:49:16: E721 do not compare types, for exact checks use `is` / `is not`, for instance checks use `isinstance()`
tests/validators/none_to_unset_value_test.py:34:16: E721 do not compare types, for exact checks use `is` / `is not`, for instance checks use `isinstance()`
tests/validators/noneable_test.py:34:16: E721 do not compare types, for exact checks use `is` / `is not`, for instance checks use `isinstance()`
tests/validators/noneable_test.py:50:16: E721 do not compare types, for exact checks use `is` / `is not`, for instance checks use `isinstance()`

I fixed the style according to these messages, now the tests run without flake8 complaining again :)

[binaryDiv]

Usually isinstance would be the way to go. However, since these are all assertions in the unit tests to check the type of the validated result, I think a strict test with type(result) is type(expected_result) would be better.

If the validator results with an object of a subclass of the expected type, something is wrong.