Epoch with merged /usr baselayout and python3

[silkeh]

The goal of this issue is to track the progress to the next epoch. The goal of this epoch is to have a repository 1) with merged /usr baselayout (see #293) and 2) without python2

Dependencies:

• Merge /usr (T10615) #293
• eopkg: Handle the py2-based pisi to py3-based eopkg migration as robustly as possible #2193
• Tracker task for epoch-related issues that need solving in eopkg eopkg#73

Transition:

• Update eopkg in old repo with functionality that checks if the system has a merged /usr and switches the repo URL.
• Create new stable repository.

Post-epoch tasks:

• Publish updated baselayout.
• Remove all python2 based packages.
• Remove compatibility symlinks from before usr-merge.