docs: sphinx-pre-install: don't barf on beta Sphinx releases

sphinx-pre-install is picky when it comes to parsing sphinx versions; it
failed when run with sphinx 4.0.0b1.  Tweak the regex to tolerate a
trailing "bN" on the version number.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>

diff --git a/scripts/sphinx-pre-install b/scripts/sphinx-pre-install
index b5f9fd5..fe92020 100755 (executable)
--- a/scripts/sphinx-pre-install
+++ b/scripts/sphinx-pre-install
@@ -259,7 +259,7 @@ sub get_sphinx_version($)
 
        open IN, "$cmd --version 2>&1 |";
        while (<IN>) {
-               if (m/^\s*sphinx-build\s+([\d\.]+)(\+\/[\da-f]+)?$/) {
+               if (m/^\s*sphinx-build\s+([\d\.]+)((\+\/[\da-f]+)|(b\d+))?$/) {
                        $ver=$1;
                        last;
                }