Change the default Python version in documents to 3 instead of 2

[jordigh]

Original commit messages:

• e513d6b workflows: install gvisor from Ubuntu repos

  This requires Ubuntu 24.04, as 22.04 doesn't package gvisor yet.

• ba0355f test: normalise the test for SQLite infinity

  Older SQLite used 1.0e999 as the printf output for an IEEE784 Inf value,

  https://sqlite.org/printf.html

  whereas newer uses 9.0e+999. The difference doesn't matter for us, so
  let's normalise to the later version.

• d88c424 ActiveDoc: make Python 3 the default version

  It's already default in various contexts, but sometimes there is
  nothing defining what the Python version, which makes it default to 2.
  This has broken some documents in the past. It's high time to change
  this default.

Context

It is sometimes possible to create Python 2 documents by default, such as older Grist Desktop. That default breaks in other contexts, such as Grist SaaS. It's best to default more strongly to Python 3. See here for a case where this affected users.

Proposed solution

Change the default to Python 3 if the PYTHON_VERSION isn't set.

Has this been tested?

• 👍 yes, I added tests to the test suite (in an earlier commit).
• 💭 no, because this PR is a draft and still needs work
• 🙅 no, because this is not relevant here
• 🙋 no, because I need help

[paulfitz]

I wonder if we'll need to tweak grist-saas? Anyway, this seems correct, Python2 as a default makes no sense anymore.