Dockerfile: Change python version

Changed python version to 3.6 to make it
compatible to igitt.

setup.cfg: Correct doc-test cli parameter

Added a missing 's' to the doc-test cli parameter

wolfram_alpha.py: Remove unused code

Removed try-except section which
was not called and added new log

wolfram_alpha_test.py: Change assert

Changed assert statement according to new
log in wolfram_alpha.py

test-requirements: Update pytest-cov version

Changed pytest-cov version to be less than 2.9
to solve dependency conflicts

Fixes #667