[3.14] gh-145455: Show output of blurb & sphinx-build version commands (GH-145457) (#145461)

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In gh-145455, an outdated dependency caused an import error that was not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.
(cherry picked from commit f1de65b3669226d563802a32b78a2294e971151a)

Co-authored-by: Petr Viktorin <encukou@gmail.com>