Python 3 tdb sometimes missed

[Douglas Bagnall]

David had to marked the GPO tests as Py3-incompatible:

>     gpo: Disable python3 testing
>     
>     The gpo module doesn't work in python3 yet,
>     causing this test to fail on python3.

I think because https://bugzilla.samba.org/show_bug.cgi?id=13525

Recent distro releases include the current tdb (1.3.15) and with it
the python 2 bindings. Waf run under python 2 decides it can use the
system bindings so it doesn't build the py3 ones.

Conversely, if python 3 ran waf, we would build build python 2 bindings
when we were really trying not to.

This is all very solvable but I'm not doing that right now.

Douglas