Avoid newer isort for flake8 tests

Do not install the latest isort because it currently breaks
flake8-isort. This pinning should be removed as soon as
gforcada/flake8-isort#88 is resolved.

tox.ini

@@ -18,6 +18,9 @@ deps = flake8
        flake8-tidy-imports
        flake8-tuple
        pep8-naming
+       # keep the following until this is fixed:
+       # https://github.com/gforcada/flake8-isort/issues/88
+       isort<5
 commands = flake8 muninplot setup.py
 
 [testenv:eslint]