Move pytest dependcy to the development dependencies

[Nicoretti]

Rational: a strictly pinned pytest should not be a non dev dependency, likely this is a left over of the removed pytest plugin.

All Submissions:

• [x] Is the title of the Pull Request correct?
• [x] Is the title of the corresponding issue correct?
• [x] Have you updated the changelog?
• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• [x] Are you mentioning the issue which this PullRequest fixes ("Fixes...")
• [x] Before you merge don't forget to run all tests for all Exasol version, by adding [run all tests] to the commit message
• [x] Are the CLI usage examples up to date? (No change)