replace py26 test configuration with py36

[lczub]

With #21 py26 support was add. Cause www.python.org supports as py2 only py27, the tox and travis test configuration will not look for py26 anymore.

Instead tox and travis should use py36 for testing. Also py33 is not needed anymore

[lczub]

problematic method TestlinkAPIGeneric._convertPostionalArgs() will still use the py26 and py27 compatible line return dict(list(zip(nameList, valueList)))

so py26 might still work, but we will not run any test on py26 anymore.

But much nicer would be return {nameList[x] : valueList[x] for x in range(len(nameList)) }
maybe in sometime in the future

[lczub]

v0.6.4