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* Adjust "internal" imports in package and tests.
* Switch to py.test to run tests in tox (better, and lgc.pytest does not like
  absolute imports apparently -- maybe because of chdir that's performed prior
  to run tests).

This is not used in this cube and should be handled in downstream application.

* Distinguish `ValidationScript` and `TransformationScript`.
* Rename `ScriptSequence` and `ScriptSequenceItem` into
* `TransformationSequence` and `TransformationStep`.

Otherwise, anonymous gets modification powers.

Previous restrictions were for datacat business logic.

Parameters are modelled as a string attribute on Script entity type. This is
simply transformed as options for the underlying Python script upon execution.

Closes #10428307.

The former will allow for storing large stderr output (e.g. log files), the
latter makes it possible for a script to change the format of its output.

The script sequence consists of sequence items, with an index corresponding to
their position in the sequence and a relation to a script.

Simplify tests in unittest_entities, drop tests for things that are not
actually used (and dropping those things as well).

Related to #10428298.

Closes #10267618.