Comment 4 for bug 1712634

This also affects raiden:
https://github.com/elopio/raiden/blob/082acd1cb76a3a71dffead08685b8ee1516b7721/snap/snapcraft.yaml#L40

The workaround there is a little different, because we need to install requirements before running setup.py. Then, I added a dependent part, with the hack to delete the setup.py on prepare.