Docs: drop Python 2 support

The kernel build system as a whole is dropping support for Python 2, so we
should do the same.  The effects are rather small, especially considering
that much of the deleted code was not doing anything under any version of
Python anyway.

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

scripts/sphinx-pre-install

@@ -728,8 +728,8 @@ sub check_needs()
 					$need_virtualenv = 1;
 				}
 				if ($1 < 3) {
-					# Complain if it finds python2 (or worse)
-					printf "Warning: python$1 support is deprecated. Use it with caution!\n";
+					# Fail if it finds python2 (or worse)
+					die "Python 3 is required to build the kernel docs\n";
 				}
 			} else {
 				die "Warning: couldn't identify $python_cmd version!";