package/python-attrs: depends on python3

python-attrs contains a file with API for python 3.6+.
Although we can make a "tweak" and remove this specific file
for python2, we prefer to make this package available only for
python3, because the day this package (and many others) will drop
support for python2 is probably very close.

In addition, update python-attrs recursive dependencies to
depend on python3 too (python-automat is the only one left).

Fixes:
 - http://autobuild.buildroot.net/results/aa6/aa6889bc254eaa93adb4fc1a71bcee9f2f23cb33/



Signed-off-by: Asaf Kahlon <asafka7@gmail.com>
[yann.morin.1998@free.fr: fix check-package]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>