Python versions and CI testing

[Douglas Bagnall]

If (as seems the case in another thread) our Centos 7 CI will use Python
3.6, will we actually have any tests using 3.4?

Debian 9 has Python 3.5, which we perhaps ought to support for a while.
In order to do that we should actually run the Python tests there,
rather than just compiling. Otherwise we'll end up in the situation we
had with Python 2.6 which we supported in theory, but in practice only
by about 4.x.5 because we broke it every time and hardly anyone noticed.

In the 4.10 release notes we said we'd soon drop Python 3.4 support, so
it is fitting for us to do so.

cheers,
Douglas