I followed everything step by step on the Heroku guide. This is what I got on the application log:
2018-05-06T16:45:33.864399+00:00 app[worker.1]: from .http import HttpResponseParser, StreamWriter
2018-05-06T16:45:33.864403+00:00 app[worker.1]: File "/app/.heroku/python/lib/python3.6/site-packages/aiohttp/http.py", line 8, in <module>
2018-05-06T16:45:33.864523+00:00 app[worker.1]: from .http_parser import (HttpParser, HttpRequestParser, HttpResponseParser,
2018-05-06T16:45:33.864528+00:00 app[worker.1]: File "/app/.heroku/python/lib/python3.6/site-packages/aiohttp/http_parser.py", line 15, in <module>
2018-05-06T16:45:33.864657+00:00 app[worker.1]: from .http_writer import HttpVersion, HttpVersion10
2018-05-06T16:45:33.864661+00:00 app[worker.1]: File "/app/.heroku/python/lib/python3.6/site-packages/aiohttp/http_writer.py", line 304, in <module>
2018-05-06T16:45:33.864920+00:00 app[worker.1]: class URL(yarl.URL):
2018-05-06T16:45:33.864925+00:00 app[worker.1]: File "/app/.heroku/python/lib/python3.6/site-packages/yarl/__init__.py", line 232, in __init_subclass__
2018-05-06T16:45:33.865399+00:00 app[worker.1]: "is forbidden".format(cls))
2018-05-06T16:45:33.865421+00:00 app[worker.1]: TypeError: Inheritance a class <class 'aiohttp.http_writer.URL'> from URL is forbidden