gh-145455: Show output of blurb & sphinx-build version commands (GH-145457)

In gh-145455, an outdated dependency caused an import error that was not
printed out (`2>&1`); the message instead said that the tools are missing.

Don't redirect stderr, to show warnings and failures.

Also, switch `blurb` to output a version on a single line (`--version` rather
than `help`), and, and don't redirect stdout either.
This results in two version info lines being printed out. These get drowned
in typical Sphinx output, and can be helpful when debugging.