Remove usage of unittest

The esasky and utils/tap modules are using unittest which triggers an error upstream when using python 3.6. The best and easiest way to deal with it in astroquery to change this test to be pure pytest.

==================================== ERRORS ====================================
_ ERROR collecting lib.linux-x86_64-3.6/astroquery/esasky/tests/test_esasky.py _
_pytest.runner:149: in __init__
    ???
_pytest.main:431: in _memocollect
    return self._memoizedcall('_collected', lambda: list(self.collect()))
_pytest.main:311: in _memoizedcall
    res = function()
_pytest.main:431: in <lambda>
    return self._memoizedcall('_collected', lambda: list(self.collect()))
_pytest.python:600: in collect
    ???
_pytest.python:454: in collect
    ???
_pytest.python:466: in makeitem
    ???
_pytest.vendored_packages.pluggy:724: in __call__
    return self._hookexec(self, self._nonwrappers + self._wrappers, kwargs)
_pytest.vendored_packages.pluggy:338: in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
_pytest.vendored_packages.pluggy:333: in <lambda>
    _MultiCall(methods, kwargs, hook.spec_opts).execute()
_pytest.vendored_packages.pluggy:595: in execute
    return _wrapped_call(hook_impl.function(*args), self.execute)
_pytest.vendored_packages.pluggy:249: in _wrapped_call
    wrap_controller.send(call_outcome)
E   DeprecationWarning: generator 'pytest_pycollect_makeitem' raised StopIteration