Remove support for Python 2

[abukaj]

As we do not test package for Python <3.8, I think backward compatibility code like:

kCSD-python/setup.py

Line 12 in d816da7

if sys.version_info.major < 3:

should be removed.

[ccluri]

Also, no version for future package is available from 3.8 onwards. What a mess.

We have to stop support for 2 entirely. I shall create a new branch for this issue.

[ccluri]

#154