scripts: sphinx-pre-install: report broken venv

After distro upgrades, the directory names for python may change.
On such case, the previously-created venv will be broken, and
sphinx-build won't run.

Add a logic to report it.
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/f117f03555b0636d2be0b68f3a23b1d3f03ccf1d.1656756450.git.mchehab@kernel.org
[jc: fixed typo in warning message]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

scripts/sphinx-pre-install

@@ -720,6 +720,12 @@ sub get_virtenv()
 		next if (! -f $sphinx_cmd);
 
 		my $ver = get_sphinx_version($sphinx_cmd);
+
+		if (!$ver) {
+			$f =~ s#/bin/activate##;
+			print("Warning: virtual environment $f is not working.\nPython version upgrade? Remove it with:\n\n\trm -rf $f\n\n");
+		}
+
 		if ($need_sphinx && ($ver ge $min_version)) {
 			return ($f, $ver);
 		} elsif ($ver gt $cur_version) {