validator: Hide typesafety part of traceback when run with py.test

[andras-tim]

This makes a bit lighter traceback on TypesafetyError.

[andras-tim]

This is too strange, I will make a thinner solution. - type mismatched decorated function calls can make trace with validator.py only, therefore the trace will be empty in this case

In other hand, there was an Travis env problem.

[andras-tim]

There was not enough to use the __tracebackhide__ = True trick, because e.g. this test with type mismatch by decorator could make empty traceback:

import pytest

@pytest.fixture
def foo():
    return []

def test_type_mismatch(foo: dict):
    assert isinstance(foo, list)