Why aiohttp==3.5.4 in requirements.txt?

[frillip]

Previously installed darksky_weather via pip3 before this requirement. At the time aiohttp was at around 3.6.1 or so, both my application and darksky_weather functioned well.

Later, I go to recreate the environment on another system, only to end up in a world of broken dependencies because this aiohttp==3.5.4 requirement has appeared from nowhere, and the other libraries required aiohttp>=3.6.0.

Installing darksky_weather and forcing aiohttp==3.6.1 over the top of that has fixed my immediate problems, by why the dependency to this old and very specific version?