Somehow python 3 came back (after some updates?) and is causing big issues with apt updates, so I've decided to remove python 3 completely from the alternatives:
root:~# python -V
Python 3.5.2
root:~# update-alternatives --config python
There are 2 choices for the alternative python (providing /usr/bin/python).
Selection Path Priority Status
------------------------------------------------------------
* 0 /usr/bin/python3.5 3 auto mode
1 /usr/bin/python2.7 2 manual mode
2 /usr/bin/python3.5 3 manual mode
root:~# update-alternatives --remove python /usr/bin/python3.5
root:~# update-alternatives --config python
There is 1 choice for the alternative python (providing /usr/bin/python).
Selection Path Priority Status
------------------------------------------------------------
0 /usr/bin/python2.7 2 auto mode
* 1 /usr/bin/python2.7 2 manual mode
Press <enter> to keep the current choice[*], or type selection number: 0
root:~# python -V
Python 2.7.12
root:~# update-alternatives --config python
There is only one alternative in link group python (providing /usr/bin/python): /usr/bin/python2.7
Nothing to configure.