Use ancestory to avoid processing code blocks.  For every node that is handled via the inline pattern, we will build up a parent list.  As we decend into recursion, we append and pop children as needed.  Though it is not practical, for this experiment, we purposely leave code tag text unprotected and default all inline patterns to avoid 'code'.

Note that we install coverage<4.0 in travis because test-requirements.txt is
installing the same in the tox environments. The .coverage file format
changed in 4.0.

Travis-CI no longer has all supported Python versions installed by default.
Each version needs to be set via the `python` setting. However, we don't
want a matrix where every Python version runs with every `TOXENV`.
Therefore, we now manually define the matrix so that the Python version
is properly matched with the TOXENV.