flatmanager: Fix nesting of context managers

appstream_gz and appstream_file are only needed for shutil.copyfileobj
so those two are joined together and added in the innermost position
while tempfile is kept in the outermost position.

Once shutil.copyfileobj is done both innermost handles are closed
automaitcally and the outermost handle can access the same resource
again

flatpak_builder_lint/checks/flatmanager.py

@@ -74,14 +74,14 @@ def check_repo(self, path: str) -> None:
                         self.errors.add("flat-manager-branch-repo-mismatch")
                         break
 
-                with (
-                    tempfile.TemporaryDirectory() as tmpdir,
-                    gzip.open(
-                        f"{path}/appstream/{arches.pop()}/appstream.xml.gz", "rb"
-                    ) as appstream_gz,
-                    open(f"{tmpdir}/appstream.xml", "wb") as appstream_file,
-                ):
-                    shutil.copyfileobj(appstream_gz, appstream_file)
+                with tempfile.TemporaryDirectory() as tmpdir:
+                    with (
+                        gzip.open(
+                            f"{path}/appstream/{arches.pop()}/appstream.xml.gz", "rb"
+                        ) as appstream_gz,
+                        open(f"{tmpdir}/appstream.xml", "wb") as appstream_file,
+                    ):
+                        shutil.copyfileobj(appstream_gz, appstream_file)
 
                     if not appstream.has_manifest_key(f"{tmpdir}/appstream.xml"):
                         self.errors.add("appstream-no-flathub-manifest-key")