Update nanobind-bazel docs with latest developments

[nicholasjng]

nanobind_bazel v2.4.0 was released on the BCR on christmas. The MODULE.bazel code example is thus updated, as is tradition, along with two more user-facing changes:

1. The minimum Bazel version was increased to Bazel 7, since we need to use a sufficiently new rules_python for free-threading support.
2. The @nanobind_bazel//:free_threading flag was removed, since it was not that useful - you still needed to give rules_python's free-threading flag separately to request a free-threaded toolchain, and supplying it did not even guarantee a free-threading build. Now, we just use rules_python's free-threading flag value directly instead.

[wjakob]

Actually, onecomment: it may be useful to keep the free-threading attribute to as an opt-in (similarly to how this is implemented in the CMake build system). Upgrading a package to correctly work in a free-threaded environment can be quite complex. One checkpoint along the way is just to run it as-is (not opted in), in which case the free-threaded features of the interpreter are turned off. Not very useful if your goal is parallelism, but helpful for bring-up of large multi-extension projects.

[nicholasjng]

I agree - in fact, rules_python's free-threading flag means that a free-threaded toolchain will be used instead of a "classic" one whenever possible (i.e. CPython 3.13+), so passing @rules_python//config_settings:is_free_threaded=no (name slightly different) just means regular CPython 3.13 will be used, and -DNB_FREE_THREADED will not be given.

Also, no free-threading is the default, so we can build regular CPython 3.13 just fine (in fact, that's what I'm still doing in nanobind_bazel CI).

Does that match your expectation?

[wjakob]

That makes sense! Is this documented somewhere? Otherwise, it may not be easy to infer for users.

[nicholasjng]

Afraid not. I can add a paragraph on free-threaded builds to the Bazel doc, either in this PR or a follow-up.

[wjakob]

Let's put it here, we can keep this PR open a little longer.

[nicholasjng]

Done.

On a side note, the docs venv setup is broken at least on my MacOS machine: When pip-installing docs/requirements.txt with Python<3.13, I get an error from sphinx about missing pkg_resources:

$ sphinx-build docs docbuild -a
Running Sphinx v6.1.3

Extension error:
Could not import extension sphinxcontrib.moderncmakedomain (exception: No module named 'pkg_resources')

That one can be fixed by running pip install --upgrade setuptools. On Python 3.13, it's another module that errors out:

$ sphinx-build docs docbuild -a
Running Sphinx v6.1.3

Extension error:
Could not import extension sphinx.builders.epub3 (exception: No module named 'imghdr')

Just wanted to put this out there, since there are other people working on the documentation as well.

EDIT: The second one is about imghdr, deprecated in Python 3.11 and subsequently removed in Python 3.13. The removal in Sphinx came in version 6.2.0, which is just a bit newer than the pin in the requirements file, source: sphinx-doc/sphinx#10440

[wjakob]

Thank you! I'll make a note to transition to a newer version of Sphinx in the future.