1.16.0 release-candidate in a plain py3 environment with system-wide installs

[jg71]

Hi,

let's collect all the issues you might have come across taking the
-rc0 for a test drive.

I'll start with Slackware64-current, plain py3 install, but
system-wide (no virtualenv foo), soon-ish.  Give me this weekend to
file this properly.  I've come across some obstacles last weekend
which I will then present.

I've since started a small test-grid eventually, but had limited time
to put it to the test. Looks good so far.

Also interesting to know for a wider audience is that the release
would apparently happen next week-ish, so let's talk about all the
nasty little things wrt plain py3 usage scenarious.

lest we forget: anyone out there still on that py2-only island?

-- jg71