gh-145455: Show output of blurb & sphinx-build version commands

In gh-145455, an outdated dependency caused an import error that was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.

Switch `blurb` to output a version on a single line (`--version` rather
than `help`), and don't redirect stderr, to show warnings and failures.
Also, don't redirect stdout. This results in two extra lines with
version info, which get drowned in typical Sphinx output, and can be
helpful when debugging.

Author: encukou

Doc/Makefile

@@ -58,7 +58,7 @@ build:
 	@if [ -f  ../Misc/NEWS ] ; then \
 		echo "Using existing Misc/NEWS file"; \
 		cp ../Misc/NEWS build/NEWS; \
-	elif $(BLURB) help >/dev/null 2>&1 && $(SPHINXBUILD) --version >/dev/null 2>&1; then \
+	elif $(BLURB) --version && $(SPHINXBUILD) --version ; then \
 		if [ -d ../Misc/NEWS.d ]; then \
 			echo "Building NEWS from Misc/NEWS.d with blurb"; \
 			$(BLURB) merge -f build/NEWS; \