We'll need persistent as git checkout in the next patch when organizing testing
of ZODB stack, because persistent tests want to discover in-tree files that are
not present in persistent egg when it is installed in non-development mode:

https://erp5.nexedi.net/test_result_module/20201123-3F859E35/7
(look for "AssertionError: could not find my setup.py")

https://github.com/zopefoundation/persistent/blob/4.6.4-0-g7ed95cf/persistent/tests/test_docs.py#L37-L43