preparing for the next archive opening

[Matthias Klose]

We usually tend to open the archive with a few prepared changes in place, for 
the 20.04 cycle we are planning to open with

  - python3.8 as a supported version, maybe already as the default.

  - link time optimization enabled by default in GCC. This will
    add a few ftbfs (~400).  I am currently planning to have a
    blacklist package listing all the source which cannot be
    built with LTO.  The alternative of having to upload around
    400 packages to disable LTO isn't appealing either.

  - new boost version

Desktop and server teams didn't want to add anything to this list yet.

When enabling the Debian syncs, we'll see some more transitions, as usual.