[pkg] version 0.7.3
Why upgrading CubicWeb minimum version? We don't support 3.25 and less, and even 3.26 should disappear but it is the last version that supports Python 2 and it is in consequence sadly still used. So it is better not to create illusion and be clear that we won't add a fix for a too old version of CubicWeb. Projects must upgrade, maintaining very old stuff is dirty work.