Since we have quite a lot of branches now, I want to list them here.
The master branch and all stable branches are for Python 3. master builds against kdevplatform master, the 5.0, 5.1 etc branches build against kdevplatform of the same version. There is no Python 2 support for KDevelop 5.
There are lots of stable branches with names like
1.3 1.4 1.5 1.6
Those are all python 2 and work with the kdevplatform version which matches the branch name.
branches are for python 3 and work with the kdevplatform version which matches the branch name.
Note that "1.6.40" or higher for example is *not* 1.6, but 1.7 alpha.