Retry decorator does not play well with other decorators

[jalaziz]

This may be related to #318, but it involves the return value rather than the type.

When using the retry decorator with ddtrace-py's tracer.wrap(), the return value of the function seems to always be None despite the function actually returning a value.

The function looks something like:

@retry(
    retry=(
        retry_if_exception_type(APIError)
    ),
    stop=stop_after_delay(120),
    wait=wait_random_exponential(multiplier=0.5, max=20)
)
@tracer.wrap()
async def get_value(self):
    return 'value'