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* Reject empty passwords by default.
Unless :setting:`AUTH_LDAP_PERMIT_EMPTY_PASSWORD` is set to True,
LDAPBackend.authenticate() will immediately return None if the password is
empty. This is technically backwards-incompatible, but it's a more secure
default for those LDAP servers that are configured such that binds without
passwords always succeed.
* Add support for pickling LDAP-authenticated users.