Make site be buildable with Python 3

[perlun]

Quoting #26 (comment), which in turn takes this from 81254d8:

There is an annoying problem here:

• The site depends on pygments.rb
• Pygments.rb in turn, communicates with a Python process using a pipe (reading and writing to stdin/stdout from Python running as a child process)
• The version of Pygments.rb we use (?) do not work with Python 3. Only Python 2 works.

Ubuntu 22.04 uses Python 3 by default (and this is perhaps the case with Ubuntu 20.04 also), which causes errors like the one seen in https://github.com/perlun/perlun.eu.org/actions/runs/5913115348/job/16037183000. Copying it here into the git commit log, since Github Actions logs are unfortunately not retained forever.

bundle exec jekyll build
      Deprecation: The 'gems' configuration option has been renamed to 'plugins'. Please update your config file accordingly.
Configuration file: /home/runner/work/perlun.eu.org/perlun.eu.org/_config.yml
           Source: /home/runner/work/perlun.eu.org/perlun.eu.org
      Destination: /home/runner/work/perlun.eu.org/perlun.eu.org/_site
Incremental build: disabled. Enable with --incremental
     Generating...
Building site for default language: "xx" to: /home/runner/work/perlun.eu.org/perlun.eu.org/_site
Building site for language: "sv" to: /home/runner/work/perlun.eu.org/perlun.eu.org/_site/sv
Loading translation from file /home/runner/work/perlun.eu.org/perlun.eu.org/_i18n/sv.yml
Building site for language: "en" to: /home/runner/work/perlun.eu.org/perlun.eu.org/_site/en
  Conversion error: Jekyll::Converters::Markdown encountered an error while converting '_i18n/en/_posts/2017-03-17-new-features-in-c-sharp-6-0-and-7-0.md':
                    No header received back.
jekyll 3.9.2 | Error:  No header received back.

We need to make this work, by the next year at the very latest, since Ubuntu 20.04 is going EOL then. It's unclear if/when GitHub will drop support for it, but let's rest assured that it will happen sooner or later.

[perlun]

See these GitHub issues for some more details:

• Liquid Exception: No header received back - Mac OS X jekyll/jekyll#2604
• resolves #152 add support for Python 3 pygments/pygments.rb#199

[perlun]

Should be fixed as of #54, I think. Let's try upgrading the CI to Ubuntu 22.04 now to see if it works. If we're really wild and crazy, we could even give 24.04 a shot (but I'm not sure GitHub supports it yet? Checked: public beta is available now: actions/runner-images#9691).

[perlun]

Let's try upgrading the CI to Ubuntu 22.04 now to see if it works

#56

[perlun]

Fixed in #56 indeed, and seems to works fine. 🎉 Closing away.