Change tempfile open mode to 'w'

- Previously was 'w+b' (default)
- Will fix #2426

desktop/core/src/desktop/lib/metrics/file_reporter.py

@@ -46,6 +46,7 @@ class FileReporter(Reporter):
     # rename the file to the real location.
 
     f = tempfile.NamedTemporaryFile(
+        mode='w',
         dir=dirname,
         delete=False)