Project now passes flake8 and black linter (also including more rst
cleanup). Moved latex related things to dedicated ``latex.py`` module,
and removed the ``makesheets -dF`` call from travis.
In order to avoid parsing arbitrary python files encountered during
directory recursion, ``makesheets`` now checks each python file for a
string listing the *dungeonsheets_version* before trying to import the
character file.
Mark Wolfman