Prepare for 0.20.0 release

[antonwolfy]

The PR implements changes required for 0.20.0 release

• Have you provided a meaningful PR description?
• Have you added a test, reproducer or referred to an issue with a reproducer?
• Have you tested your changes locally for CPU and GPU devices?
• Have you made sure that new changes do not introduce compiler warnings?
• Have you checked performance impact of proposed changes?
• Have you added documentation for your changes, if necessary?
• Have you added your changes to the changelog?

[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/index.html

[coveralls]

Coverage is 78.429% — update-dependent-versions into maintenance/0.20.x. No base build found for maintenance/0.20.x.