How to prepare a new release

Update the version number in the following files:
- pyproject.toml
- litebird_sim/version.py
- docs/source/conf.py
- CHANGELOG.md (be sure to leave an empty HEAD title at the top);
Commit all the changes to master.
Build the release:
```
poetry build
```
Upload the package to PyPI:
```
poetry upload
```

Create a new tag and push it to GitHub:

git tag -a vX.Y.Z -m "Version X.Y.Z"
git push origin vX.Y.Z

Open the page https://github.com/litebird/litebird_sim/releases and create a new release from the tag you have just created.
Use the script prepare_release_email.py to automatically produce the text of an email containing the release notes:
```
$ python3 prepare_release_email.py | xclip -selection clipboard
```
and send this text to the LiteBIRD JSGs.