docs/sphinx/depfile.py: Handle env.doc2path() returning a Path not a str

In newer versions of Sphinx the env.doc2path() API is going to change
to return a Path object rather than a str. This was originally visible
in Sphinx 8.0.0rc1, but has been rolled back for the final 8.0.0
release. However it will probably emit a deprecation warning and is
likely to change for good in 9.0:
  https://github.com/sphinx-doc/sphinx/issues/12686

Our use in depfile.py assumes a str, and if it is passed a Path
it will fall over:
 Handler <function write_depfile at 0x77a1775ff560> for event 'build-finished' threw an exception (exception: unsupported operand type(s) for +: 'PosixPath' and 'str')

Wrapping the env.doc2path() call in str() will coerce a Path object
to the str we expect, and have no effect in older Sphinx versions
that do return a str.

Cc: qemu-stable@nongnu.org
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2458
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240729120533.2486427-1-peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
(cherry picked from commit 48e5b5f994)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

docs/sphinx/depfile.py

@@ -19,7 +19,7 @@ __version__ = '1.0'
 
 def get_infiles(env):
     for x in env.found_docs:
-        yield env.doc2path(x)
+        yield str(env.doc2path(x))
         yield from ((os.path.join(env.srcdir, dep)
                     for dep in env.dependencies[x]))
     for mod in sys.modules.values():