Bug#1055488: please build for all supported Python3 versions

[Matthias Klose]

Control: reopen -1
Control: severity -1 important

with the recent upload to only build for the default Python3 version, at 
least the build of the package onboard fails, because it tries to build 
for all supported Python3 versions.

Please also build pycairo for all supported Python versions, or else we 
will only see issues with a new Python version in the gtk/gnome stack 
when we change the Python3 default.