[debian] Introduce python3-cubicweb and python3-cubicweb-pyramid binary packages This is the bare minimum to have a working cubicweb on python3. Further work is done in default branch.

[skeleton] Update Debian packaging template Here are multiple updates to the Debian packaging template provided when running `cubicweb-ctl newcube`: * Switch to source format 3.0 (quilt). * Switch to debhelper 9. * Switch to dh-python. * Switch to Python 3. * Name the binary package `python3-cubicweb-CUBENAME` instead of just `cubicweb-CUBENAME` (which is still the source package). * Populate Depends using dh_python3 support for Python requirements. * Run test suite at build time using pytest. * Add autopkgtest to run test suite against the installed package. * Bump Standards-Version to 4.3.0.

[test] Use --short-description when testing newcube command

[devtools] Allow to specify short desc on `newcube` command line In order to implement automated testing of `cubicweb-ctl newcube`, we need to avoid any user interaction. So we add a `-s` switch to specify a short description on the command-line instead of requiring an input.

[pkg] Remove build dependency on dh-systemd As suggested by Lintian, remove build dependency on obsolete package dh-systemd. Instead, specify a more precise version of debhelper 9 that added support for systemd.

[config] Raise in case of failure to send emails when in test mode Since, for some reason, exception log messages are not visible when running tests, it's very hard to debug when sendmails() method fails during tests. So, as in some other places, let's raise the exception when in test mode.