Univention Corporate Server 4.4 erratum 630 (bugfix)

This update addresses the following issues:
* The code base has been converted to Python 3.
* Invalid Python string literals are now detected.
* UCS templates are checked for compatibility with Python 2 and 3.
* `debian/compat` is now checked for consistency with the declared versioned
  build dependency of `debhelper` in `debian/control`.
* `debian/*.dirs` is now checked for un-needed entries which are already
  created indirectly by other steps.
* New `debhelper` related files in `debian/` are recognized.
* `debian/*.ucs files are checked more strictly due to the switch to Python
  3. For example duplicate keys are now errors.
* The `debian/changelog` file is checked for strict-monotonic entries. In the
  past this has lead to surprising update results as the timestamp of the
  latest entry is used for many things during the package build.
* Un-join-script files are now checked for errors, too.
* Debian maintainer script `debian/*.{pre,post}{inst,rm}` are now checked for
  handling wrong actions.
* `ucslint` has been removed as a direct dependency from all UCS packages as
  it now runs directly from our CI pipeline.
* Check for usage of `uldap.searchDn()`.
* Check for packages declaring dependencies on `Essential:yes` packages.
* Check users of `custom_{user,group}name()` registering for required UCR
  variables.
* Check for usage of `debian/*.pyinstall` files to install Python modules.