Remove python 2.7 testing

[tdamsma]

Python 2.7 support is now dropped for 0.25, but the CI tests are still run against python 2.7

[mroeschke]

#24942 is already in motion converting the 2.7 builds to 3.5 compatibility builds.

[tdamsma]

I really tried to look for a similar issue but somehow missed it. Will close