fixes docs creation with readthedocs

The changes implmented in this PR together with the RTD options ``Use System Packages`` and ``CPython 2.x`` as interpreter fix the current problems.

Note that in particular code like in https://github.com/xrotwang/lingpy/blob/c4139758c06d2f99ee071f1c2eb9d2dc5ccec94f/lingpy/data/ipa/sampa.py is problematic for RTD or any tool that will import all modules, because all the code is run.