Not sure actually. I had this issue and just uninstalled 3.5 and 3.6 that were on my Win10 PC. Installed fresh 3.8.1, changed PATHs, upgraded
pip and ran flexget install command. Then after half a day or so RAM starts getting eaten and after a day it's all gone and starts getting into page file I think, because my HDD works like crazy and if I'm able to, I just kill the Python task - if not, because it becomes even impossible to move the mouse, press reset button. Downgraded back to 3.6.8 (the latest 3.6 I could find executable) and it's back to normal.
Here's my tasks schedule (you can guess what each task is for):
BTW, 16GB of RAM on my PC