Release new version 1.6.3 on PyPI

[bittner]

A stable version of Autograd was last released on PyPI on 23 June 2023 (commit 1bb5cbc). Afterwards, a few investments were made that make sure Autograd can continue to be installed successfully (e.g. #602). Those changes are still unreleased.

It would help if a new release on PyPI were made.

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P.S.: As explained in #605 (comment), the only thing that needs to be done is push a Git tag in order to trigger the automatic release process. As a one-time prerequisite, credentials from PyPI need to be configured on GitHub beforehand.