Drop python2

There is no support for python2 from upstream anymore, so we should get rid of it in our remaining packages:

community

• chromium
• nodejs8
• qt5-qtwebengine

testing

• node-libpg-query (!31016 (merged))
• proot (checkdepends - runs with 3)
• pypy (!26176 (merged))
• qt6-qtwebengine
• seamonkey (removed)
• py2-setuptools/python2 (last to go)
• rdiff-backup1 (maybe should be dropped? 2.x exists)