Disco Dingo is now open for development

[Matthias Klose]

Disco Dingo is now open for development, with the syncs from unstable
done and built. The development version starts with only a few changes:

- Python 3.7 is now the default Python3 version, and the only supported
Python3 version.

- Perl was updated to version 5.28

- Merged-usr is now the default in Disco for new installations only.
It means that /bin is a symlink to usr/bin, similarly /lib and /sbin.
Existing systems, upon upgrade, will not be reconfigured for
merged-usr. Care needs to be taken to ensure that both merged-usr and
split-usr systems are continued to be supported. For example apparmor
rules may need to be adjusted to use {,/usr}/bin/touch and similar.
Please use debootstrap from cosmic or later, until the pending SRUs
are approved.

- OpenSSL 1.0 removal. It is expected that OpenSSL 1.0 will be removed
from Disco release, with OpenSSL 1.1.1 LTS as the only provided
OpenSSL ABI.

Please check your uploads in a disco chroot, don't just test in a cosmic or
bionic environment. See [1 or [2] how to setup such a development chroot.