Recently we have been adding some tests which use the new tmp_path
fixture from pytest, this is new in 3.9.

Since Thomas Nixon fixed the datafiles plugin to be compatible
with pytest >= 3.8.0, we can now depend on it and avoid the
warnings and errors which the older datafiles plugin causes.

This fixes issue #636

After having fixed the more recent linting errors, the
tests should pass with recent versions of pylint. Tested
with pylint 2.2.2.

Misc fixes to the tests and newer linter errors

Closes #636

See merge request !1011