Bump the isort test dependency version

This repo uses requirements-builder to test against both minimum and latest
versions of dependencies. However, isort 4.x sorts imports inconsistently
compared to isort 5.x, which means CI can't currently pass.

Bumping the version forces all test runs to use the same version of isort,
until later versions are released.

setup.py

@@ -24,7 +24,7 @@
     'invenio-access>=1.2.0',
     'invenio-accounts>=1.2.0',
     'invenio-admin>=1.2.0',
-    'isort>=4.3.4',
+    'isort>=5.6.4',
     'mock>=1.3.0',
     'pydocstyle>=1.0.0',
     'pytest-cov>=1.8.0',