[LNT, NFC] Open tmp file in a context

[thopre]

Summary

Simplify opening tmp file by using a context to have automatic closing
of the file descriptor.

[PrzemekWirkus]

LGTM but someone else must approve.

[PrzemekWirkus]

LGTM

[thopre]

Committed as 156c369ca77bdd115b458096f47a6704e65b797e

lnt/util/ImportData.py

except OSError:
         pass
 
     # Save the file under a name prefixed with the date, to make it easier
     # to use these files in cases we might need them for debugging or data
     # recovery.
     prefix = utcnow.strftime("data-%Y-%m-%d_%H-%M-%S")
     fd, path = tempfile.mkstemp(prefix=prefix, suffix='.json',
                                 dir=str(tmpdir), text=True)
-    fp = os.fdopen(fd, "w")
+    with os.fdopen(fd, "w") as fp:
         fp.write(data)
-    fp.close()
 
     # Import the data.
     #
     # FIXME: Gracefully handle formats failures and DOS attempts. We
     # should at least reject overly large inputs.
 
     result = lnt.util.ImportData.import_and_report(
         config, db_name, db, session, path, '<auto>', ts_name,
         select_machine=select_machine, merge_run=merge_run)
     return result