Remove useless dependencies in dependencies.txt

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There are some useless dependencies mentioned in dependencies.txt. At least, the COS libraries should be removed. Maybe others (idna, jmespath, lomond, six, and numpy ?).

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Actually, Vincent's point was that this file should only list the top-level packages, those that must be explicitly installed. The others will come manually.

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According to these sources, requirements.txt should really have all the dependencies, even indirect, and pinned down to the version that is known to work:

• https://medium.com/knerd/best-practices-for-python-dependency-management-cc8d1913db82
• https://stackoverflow.com/a/65666949/4177
• https://www.reddit.com/r/Python/comments/j3ilta/best_practice_for_requirementstxt_and_setuppy/
• https://realpython.com/lessons/requirements-files-best-practices/

The third one mentions setup.py, which maybe I should create in order to facilitate the use of dowml as a library.

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Created https://github.com/IBMDecisionOptimization/dowml/issues/15.