pygobject: Plan to depend on a new pycairo fork

[Christoph Reiter <reiter christoph gmail com>]

Hey,

I'm trying to establish a pycairo fork as a new upstream and I'm planing to
depend on it in upcoming versions of pygobject:

    https://pycairo.readthedocs.io
    https://github.com/pygobject/pycairo
    https://github.com/pygobject/pycairo/releases

For the cairo mailing list thread see:

    https://lists.cairographics.org/archives/cairo/2017-April/027919.html

> From a distributions POV the main changes are that it uses distutils as build
system, can build Python 2 and Python 3 versions from the same source, and is
licensed under MPLv1.1+LGPLv2.1. The new versions are API/ABI compatible to
the 1.10.0 releases of both py2cairo and py3cairo. This includes the patched
version of py3cairo currently in Debian/Ubuntu.

One issue with Debian is that it still uses py2cairo 1.8.8 and py2cairo broke
ABI between 1.8.8 and 1.8.10 and all other distributions have moved on to
1.10.0 by now. There are 8 packages directly affected by the ABI break:
https://codesearch.debian.net/search?q=Pycairo_IMPORT I assume a transition is
required here.

Depending on a newer version would require pycairo to be added to jhbuild, as
it is currently a sysdep. Since the new versions is distutils based, this is
straight forward.

Distributions building the Python 2 version with xpyb support (Fedora for
example) can pass "--enable-xpyb" to "setup.py build" to enable it.

MSYS2 has switched to the new version:
    https://github.com/Alexpux/MINGW-packages/pull/2390
Emmanuele Bassi has switched gnome-continuous:
    https://git.gnome.org/browse/gnome-continuous/commit/?id=80ce62a6

I'd appreciate any feedback and I encourage everyone to switch to this new
version and report any problems on the github bug tracker.