Skip tests for ldapsource with python >= 3.7
Until someone works on fixing these, this should make our CI green
again.
I tried to use setupModule() to check for python version, but
pre_setup_database() is apparently called even when a SkipTest exception
is raised there. So handle this in that method.
(grafted from 4d68d20427dee4b6751a0f1f5511fec2a04f4782)