Some readthedocs jobs recently started failing with a complaint from virtualenv about --no-site-packages (readthedocs/readthedocs.org#6725). It seems readthedocs passes this flag unless --system-site-packages is requested. From the logs it seems there used to be a flag by this name, and then its behavior was made the default (around 2012?), but the flag was kept. Did it recently get removed? I can't quite nail it by grepping the logs, but this code has apparently been in readthedocs for ages.

Python 3.7 on Linux.