docs: Fix the docs build with Sphinx 6.0

Sphinx 6.0 removed the execfile_() function, which we use as part of the
configuration process.  They *did* warn us...  Just open-code the
functionality as is done in Sphinx itself.

Tested (using SPHINX_CONF, since this code is only executed with an
alternative config file) on various Sphinx versions from 2.5 through 6.0.
Reported-by: Martin Liška <mliska@suse.cz>
Cc: stable@vger.kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/sphinx/load_config.py

@@ -3,7 +3,7 @@
 
 import os
 import sys
-from sphinx.util.pycompat import execfile_
+from sphinx.util.osutil import fs_encoding
 
 # ------------------------------------------------------------------------------
 def loadConfig(namespace):
@@ -48,7 +48,9 @@ def loadConfig(namespace):
             sys.stdout.write("load additional sphinx-config: %s\n" % config_file)
             config = namespace.copy()
             config['__file__'] = config_file
-            execfile_(config_file, config)
+            with open(config_file, 'rb') as f:
+                code = compile(f.read(), fs_encoding, 'exec')
+                exec(code, config)
             del config['__file__']
             namespace.update(config)
         else: