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.
if repo.system_source.dbdriver == 'postgres':
sql('ALTER TABLE appears ADD COLUMN weight float')
sql('UPDATE appears SET weight=1.0 ')