2019Q4 packaging update

[Dridi Boukelmoune]

Greetings,

Today I back-ported patches that have been living happily in the
weekly branch and now we only rely on Python3 dependencies also
for 6.x releases.

In the process I tried Debian 10 and RHEL 8 and we can now build
Varnish 6.3 on those systems without any obvious problems. The
devel might be in the details, like for example difference in memory
footprint with an updated jemalloc, or for that matter any other
dependency update on those platforms.

Currently we can't build 6.0 LTS releases on RHEL 8 because
/usr/bin/python is no longer provided by the distribution, on purpose,
so any script using plain unversioned python in its shebang will fail
to run at either build or run time.

This work was done for the 6.3 release so it's only a matter of
back-porting existing patches to 6.0 and while we could get away with
appending a '3' to our shebangs we would miss some of the Python2
cleanup that was done in the process.

Dridi