Release procedure for shub

The GitHub Actions build is configured to release shub to PyPI whenever a new tag (starting with v, e.g. v2.13.0) is committed.

The steps to do a release are:

Install bumpversion
Make sure you're at the tip of master, and then run:
```
bumpversion VERSION_PART
```
In place of VERSION_PART, use one of patch, minor or major, meaning the part of the version number to be updated.

This will create a new commit and tag updating the version number.
Push the changes and the new tag to trigger the release:
```
git push origin master --tags
```
Once the build finishes, run pip install shub in a temporary virtualenv and make sure it's installing the latest version.
Update the release information at:
```
https://github.com/scrapinghub/shub/releases
```
The GitHub action will automatically create a release draft and attach the platform-specific binaries (built with the freeze tox environment) to it.

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