Adds Apache Beam to the list of participating projects.

[tvalentyn]

If you're adding a project to the Python 3 statement, please ensure you have:

• Discussed dropping Python 2 support with all maintainers.
• Discussed signing the statement with all maintainers. See: voting thread for links to discussion and vote result.
• Added a suitable logo if the project has one
  • Approximately square logos generally fit better than banners
  • Compressed the logo to save bandwidth (run scripts/squash-images.sh on it)
• Checked the Netlify preview made from the pull request

[tvalentyn]

Thanks a lot for setting up and maintaining python3statement.org!
Do I have to add the autogenerated sg links in this pull request? If so, are there instructions somewhere? Our code lives at: https://github.com/apache/beam.

[takluyver]

Thanks! As you're linking to a non-Github site, I think it would be helpful if you could manually add the comment with the Github URL - there's a script which can use this to update the star counts and rearrange projects.

[tvalentyn]

Done, thanks.
I got "API rate limit exceeded" when running restar.py, so I did not update counters for other projects in this PR.

[takluyver]

Thanks. You don't need to run the star-count scripts as part of adding a project. :)