Add ldapom to requirements

ldapom is a Python LDAP Object Mapper using python-ldap
Migrate from ldaplib.py to ldapom and new DIT

ldapom seems to do the job well.

FedeRez has a new LDAP DIT, thus the modifications related to group
membership.
Fix: Save org modification when promoting
Maintain restrictive permissions for .pyc files

Especially useful when settings_local.py is not world-readable.