Add GitHub Actions workflow for releasing to PyPI

[HugoKlepsch]

• This workflow is started manually by creating a "release" in GitHub.
• The action uses the release environment. Configure that environment so that only the desired users (eg. repo owner) can invoke the action.
• Set up a trusted publisher so that PyPi knows that this workflow is allowed to publish. No user/password or API keys required! https://docs.pypi.org/trusted-publishers/adding-a-publisher/
• I tested this on my fork, it seems to work. I needed to add a pyproject.toml to override the configuration you have in setup.py though: https://pypi.org/project/retrying-hk/

[HugoKlepsch]

This is actually ready now 😅
I selfishly want this to go in because I want to get rid of the pinned version of retrying==1.3.4 in my projects.

[HugoKlepsch]

Oh, I took screenshots to help with repo setup in GitHub. Here they are:

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Adding the release environment, which is used to control who can run (or approve a run of) the workflow:

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The environment should be configured to require a reviewer. In the screenshot it is me, but you should put just yourself:

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Here is what it looks like to approve the workflow run after creating a release:

[groodt]

LGTM