DashboardBuilders: add notes on Python 3 version

As suggested at https://go.dev/issue/80220#issuecomment-5350598333,
it can be helpful to say a few more words about the exact Python 3
version that should be used. Also, as an optional reference, point
to a page where Python versions of existing builders can be looked
up.

For golang/go#80220.

Change-Id: I802b15a80fd348f51387a47e3e5093527171f9e5
Reviewed-on: https://go-review.googlesource.com/c/wiki/+/818400
Reviewed-by: Eric Grosse <grosse@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
diff --git a/DashboardBuilders.md b/DashboardBuilders.md
index a516142..f19b140 100644
--- a/DashboardBuilders.md
+++ b/DashboardBuilders.md
@@ -23,6 +23,8 @@
   - 20GB disk space is ideal.
   - Preferably with 2 or more (V)CPUs.
 - Python 3 installed and available in `PATH` (or equivalent) under the name `python3` (or `python3.exe` on Windows).
+  - Any reasonably modern version of Python 3 should work.
+  - Some versions of Python 3 that are too old or too new may not be as well-tested, and you may uncover problems. Consider looking up the exact Python 3 versions already being used in the [luci.golang.ci-workers](https://chromium-swarm.appspot.com/botlist?c=id&c=task&c=os&c=python&c=status&d=desc&f=pool%3Aluci.golang.ci-workers&k=pool&s=python) pool (you may be prompted to sign in to view that page), as those are certain to be well-tested.
 - Permissions
   - The bot should be run as the `swarming` user (without root rights).
   - The bot automatically updates itself. It should have permissions to do so.