Use of f-strings is not compatible with python 3.5

[brendonmackenzie]

Bumped in to the use of f-strings relatively early on. I am using Ubuntu 16.04 which has python 3.5 by default, which does not support f-strings.
I raised a couple of pull requests for chapter 04-asyncio but haven't yet gone further.

Using grep, I located additional files that may have issues:
grep -rnw . -e "f[\'\"]"

./src/09-built-on-asyncio/the_trio/prod_sync.py:24: print(colorama.Fore.YELLOW + f" -- generated item {idx}", flush=True)
./src/09-built-on-asyncio/the_trio/prod_asyncio.py:32: print(colorama.Fore.YELLOW + f" -- generated item {idx}", flush=True)
./src/09-built-on-asyncio/the_trio/prod_trio.py:29: print(colorama.Fore.YELLOW + f" -- generated item {idx}", flush=True)
./src/10-async-web/acityscape_api/config/settings.py:6: file = os.path.join(os.path.dirname(file), f"{mode}.json")
./src/10-async-web/acityscape_api/services/location_service.py:16: key = f'{zip_code}, {country}'
./src/10-async-web/acityscape_api/services/location_service.py:29: city_data = data.get(f'{zip_code}, {country}', dict())
./src/10-async-web/cityscape_api/config/settings.py:6: file = os.path.join(os.path.dirname(file), f"{mode}.json")
./src/10-async-web/cityscape_api/services/location_service.py:14: key = f'{zip_code}, {country}'
./src/10-async-web/cityscape_api/services/location_service.py:26: city_data = data.get(f'{zip_code}, {country}', dict())
./src/04-asyncio/producer_consumer/prod_async/async_program.py:32: #print(colorama.Fore.YELLOW + f" -- generated item {idx}", flush=True)
./src/04-asyncio/producer_consumer/prod_sync/sync_program.py:23: #print(colorama.Fore.YELLOW + f" -- generated item {idx}", flush=True)
./src/05-threads/basic_threads/threaded_prod.py:44: print(colorama.Fore.YELLOW + f" -- generated item {idx}", flush=True)
./src/05-threads/basic_threads/sync_prod.py:25: print(colorama.Fore.YELLOW + f" -- generated item {idx}", flush=True)

[mikeckennedy]

Thanks, see my comments on the first PR (merged).