* Vendor DataLoader from aiodataloader and also move get_event_loop behavior from `__init__` to a property which only gets resolved when actually needed (this will solve PyTest-related to early get_event_loop() issues)
* Added DataLoader's specific tests
* plug `loop` parameter into `self._loop`, so that we still have the ability to pass in a custom event loop, if needed.
Co-authored-by: Erik Wrede <erikwrede2@gmail.com>