Thanks @hugovk for the PR, and @AlexWaygood for merging it 🌮🎉.. I'm working now to backport this PR to: 3.11, 3.12.
🐍🍒⛏🤖

GH-110637 is a backport of this pull request to the 3.12 branch.

GH-110638 is a backport of this pull request to the 3.11 branch.

FWIW, 3 doesn't seem to be the actual indentation level used by rst; it just happens to be one of the most commonly used -- because of directives.

.. directive:
   This text should be indented under the 'd' of directive, so 3 spaces

* This is a list

  * nested lists must be indented under the 'T' above, so 2 spaces

Some other things can be indented with any number of spaces >= 1. Setting 3 as a default is still a good idea.

Note: it's actually a bit more complicated than that. For directives you can also use a different number of spaces as long as you are consistent within the block, whereas lists seems to break more easily. Depending on the combination of what you are using, how you are indenting, and the presence of empty lines between different sections, different things might happen, some of which are not reported as error and, depending on the theme used, might look ok in the final render.