Infra: make htmllive: open browser when ready

[hugovk]

Like python/devguide#1233.

When running make htmllive, this will open the browser when the docs have been built the first time.

It means you don't need to hunt around for the link in:

❯ make htmllive
venv already exists.
To recreate it, remove it first with `make clean-venv'.
.venv/bin/sphinx-autobuild -b html -j 8  --re-ignore="/\.idea/|/venv/|/pep-0000.rst|/topic/" --open-browser --delay 0 peps build
[sphinx-autobuild] > sphinx-build -b html -j 8 /Users/hugo/github/peps/peps /Users/hugo/github/peps/build
Running Sphinx v7.2.6
loading pickled environment... done
building [html]: targets for 1 source files that are out of date
updating environment: 0 added, 2 changed, 5 removed
reading sources... [100%] topic/typing
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... done
copying assets... copying static files... done
copying extra files... done
done
writing output... [100%] topic/typing
generating indices... done
writing additional pages... done
dumping object inventory... done
build succeeded.

The HTML pages are in build.
[I 231130 18:08:54 server:335] Serving on http://127.0.0.1:8000
[I 231130 18:08:54 handlers:62] Start watching changes
[I 231130 18:08:54 handlers:64] Start detecting changes
[I 231130 18:08:54 handlers:135] Browser Connected: http://127.0.0.1:8000/
[I 231130 18:08:55 handlers:82] Ignore: /Users/hugo/github/peps/peps/pep-0418/clockutils.py
[I 231130 18:08:56 handlers:92] Reload 1 waiters: None
[I 231130 18:08:57 handlers:135] Browser Connected: http://127.0.0.1:8000/

I'll make the same changes for the CPython repo afterwards.

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📚 Documentation preview 📚: https://pep-previews--3558.org.readthedocs.build/

[JacobCoffee]

I added this for some of my projects after finding it. It's great.