[FIX] runbot_merge: tarfile deprecation warning

Starting in 3.12 `TarFile.extact(all)` can take a `filter` parameter
which alters the behaviour of the extractor as it relates to the
application of tar metadata. Not passing this parameter is deprecated
unless the also new `TarFile.extraction_filter` attribute is set.

Now there are a few wrinkles here:

1. Both parameter and attributes were added in older patch releases
   e.g. 3.9.17 and 3.10.12 and 3.11.4.
2. The attribute form will *not* resolve string filters, it needs the
   callables.

As such just to be on the safe side of things set the attribute using
a `getattr`, in releases with the feature it will be set to a proper
value (the future default which ignores most or all metadata from the
archive), and in releases without it just sets the attribute to `None`
which should do no harm.

runbot_merge/models/patcher.py

@@ -403,6 +403,7 @@ class Patch(models.Model):
             if read:
                 out = stack.enter_context(archiver.archive(parent, *read))
                 tf = stack.enter_context(tarfile.open(fileobj=out.stdout, mode='r|'))
+                tf.extraction_filter = getattr(tarfile, 'data_filter', None)
                 tf.extractall(tmpdir)
             patch = subprocess.run(
                 ['patch', f'-p{prefix}', '--directory', tmpdir, '--verbose'],