tests: fix usage of transaction.non_atomic_requests

[blueyed]

This currently fails with Django 2.1 (beta), and it looks like the
decorator should be used just as is in general?!

NonAtomicDBTransactionAPIExceptionTests.test_api_exception_rollback_transaction_non_atomic_view:

../django/django/db/transaction.py:277: in _non_atomic_requests
    view._non_atomic_requests.add(using)
E   AttributeError: 'function' object has no attribute '_non_atomic_requests'

During handling of the above exception, another exception occurred:
tests/test_atomic_requests.py:149: in test_api_exception_rollback_transaction_non_atomic_view
    response = self.client.get('/')
../django/django/test/client.py:527: in get
    response = super().get(path, data=data, secure=secure, **extra)
../django/django/test/client.py:339: in get
    **extra,
../django/django/test/client.py:414: in generic
    return self.request(**r)
../django/django/test/client.py:477: in request
    response = self.handler(environ)
../django/django/test/client.py:140: in __call__
    response = self.get_response(request)
../django/django/core/handlers/base.py:81: in get_response
    response = self._middleware_chain(request)
../django/django/core/handlers/exception.py:37: in inner
    response = response_for_exception(request, exc)
../django/django/core/handlers/exception.py:87: in response_for_exception
    response = handle_uncaught_exception(request, get_resolver(get_urlconf()), sys.exc_info())
../django/django/core/handlers/exception.py:35: in inner
    response = get_response(request)
../django/django/utils/deprecation.py:91: in __call__
    response = response or self.get_response(request)
../django/django/core/handlers/exception.py:37: in inner
    response = response_for_exception(request, exc)
../django/django/core/handlers/exception.py:87: in response_for_exception
    response = handle_uncaught_exception(request, get_resolver(get_urlconf()), sys.exc_info())
../django/django/core/handlers/exception.py:35: in inner
    response = get_response(request)
../django/django/utils/deprecation.py:91: in __call__
    response = response or self.get_response(request)
../django/django/core/handlers/exception.py:37: in inner
    response = response_for_exception(request, exc)
../django/django/core/handlers/exception.py:87: in response_for_exception
    response = handle_uncaught_exception(request, get_resolver(get_urlconf()), sys.exc_info())
../django/django/core/handlers/exception.py:35: in inner
    response = get_response(request)
../django/django/utils/deprecation.py:91: in __call__
    response = response or self.get_response(request)
../django/django/core/handlers/exception.py:37: in inner
    response = response_for_exception(request, exc)
../django/django/core/handlers/exception.py:87: in response_for_exception
    response = handle_uncaught_exception(request, get_resolver(get_urlconf()), sys.exc_info())
../django/django/core/handlers/exception.py:35: in inner
    response = get_response(request)
../django/django/utils/deprecation.py:91: in __call__
    response = response or self.get_response(request)
../django/django/core/handlers/exception.py:37: in inner
    response = response_for_exception(request, exc)
../django/django/core/handlers/exception.py:87: in response_for_exception
    response = handle_uncaught_exception(request, get_resolver(get_urlconf()), sys.exc_info())
../django/django/core/handlers/exception.py:35: in inner
    response = get_response(request)
../django/django/core/handlers/base.py:128: in _get_response
    response = self.process_exception_by_middleware(e, request)
../django/django/core/handlers/base.py:126: in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
../django/django/views/decorators/csrf.py:54: in wrapped_view
    return view_func(*args, **kwargs)
../django/django/views/generic/base.py:68: in view
    return self.dispatch(request, *args, **kwargs)
../django/django/utils/decorators.py:42: in _wrapper
    bound_method = dec(bound_method)
../django/django/db/transaction.py:285: in non_atomic_requests
    return _non_atomic_requests(using, DEFAULT_DB_ALIAS)
../django/django/db/transaction.py:279: in _non_atomic_requests
    view._non_atomic_requests = {using}
E   AttributeError: 'method' object has no attribute '_non_atomic_requests'

Initially added in c292b3d by @hellysmile.

[blueyed]

Coverage says its fine.

https://codecov.io/gh/encode/django-rest-framework/pull/6043

Would be nice to have Codecov status integration btw.
E.g. for #6038 it would warn about not being covered fully then (see https://codecov.io/gh/encode/django-rest-framework/pulls).

[cjerdonek]

FYI, the Django issue that caused this report should be fixed in the next release:
django/django#10091

[blueyed]

@cjerdonek
Thanks!