[gentoo-dev] Migrate away from python-2 or not

[Benda Xu <heroxbd@gentoo.org>]

Dear all,

Bug 684962 (dev-python/ipython-7.5.0: package conflicts) has
demonstrated a painful consequence when upstream start to release
python3 only versions.

Upstream has dropped python-2.7 support in dev-python/ipython-7.5.0,
thus there is no python_targets_python2_7 USE flag for the ebuild.
dev-python/qtconsole, a dependant of dev-python/ipython, still supports
python-2.7.  When qtconsole get emerged with
USE="python_targets_python2_7 python_targets_python3_6" for example, old
dev-python/ipython-5.8.0-r1 is drawn, resulting in conflict against
dev-python/ipython-7.5.0.  USE=python_targets_python2_7 had to be
removed from dev-python/qtconsole to avoid it.

If one package drops python-2.7, all its dependants have to drop
python-2.7 even if they can work with python-2.7.


Given the python-2 countdown deadline being 2020-01-01, a month away,
shall we get rid of python-2?


If the answer is yes, we will need to decide on the following
python-2-only packages.

,----
| $ comm -23 <(equery -qC h python_targets_python2_7 | sort ) <(equery -qC h python_targets_python3_6 | sort) 
| dev-lang/yasm-1.3.0
| dev-libs/libxslt-1.1.33-r1
| dev-python/backports-functools-lru-cache-1.5
| dev-python/enum34-1.1.6-r1
| dev-python/functools32-3.2.3
| dev-python/futures-3.2.0
| dev-python/pygobject-2.28.6-r55
| dev-python/pygtk-2.24.0-r4
| dev-python/subprocess32-3.2.7
| dev-util/boost-build-1.70.0
| dev-vcs/subversion-1.12.2
| gnome-base/libglade-2.6.4-r2
| net-analyzer/nmap-7.70
| sys-devel/clang-8.0.1
| x11-wm/xpra-2.4.3
`----

If the answer is no, to avoid holding back new versions having only
python3, such as bug 671796 for dev-python/matplotlib bump, old versions
with python_targets_python2_7 and new versions without should be
co-installable into different SLOTs.

What do you think?

Yours,
Benda