tests: removal of isort
tests: removal of isort

* Removes isort from testing utilities as it has different behaviour
  across Python versions.

Signed-off-by: Sami Hiltunen <ebafb54bedf33e7dd12950a4634d6a0f6ba63e44@cern.ch>

inveniosoftware/invenio-theme,inveniosoftware/invenio-theme,inveniosoftware/invenio-theme,inveniosoftware/invenio-theme