dev-python/python-ldap: bump to 3.1.0, add Python 3, doc

dev-python/python-ldap: bump to 3.1.0, add Python 3, doc

* Do not override the value of libs because we do not actually link with
  them: https://github.com/python-ldap/python-ldap/issues/224
  This also means libldap_r is now used by default (it was only selected with
  USE="sasl ssl").
* Remove outdated messages related to openldap-2.3.x and python-ldap-2.3.9.
* Fix dependencies, add slot operator to openldap.
* Run all the tests which do not require slapd.
* Bump to EAPI 7.

Package-Manager: Portage-2.3.40, Repoman-2.3.9