Make pickle saving safer

Yes, I just hand-restored my corrupted pickle due to ENOSPC :-)

fosdem_video_progress.py

@@ -8,6 +8,7 @@ from dataclasses import dataclass
 from collections import defaultdict
 from pathlib import Path
 import traceback
+import os
 
 api_url = r'https://review.video.fosdem.org/api/v1/event/1/overview'
 poll_rate = 10*60 # seconds
@@ -59,7 +60,8 @@ def main():
 			traceback.print_exc()
 
 		# Save the pickle
-		# Should probably save elsewhere and do atomic rename, but whatever.
+		# Keep the previous pickle in case we fail (e.g. ENOSPC)
+		os.rename(pickle_file, pickle_file.with_suffix('.bak'))
 		with open(pickle_file, 'wb') as f:
 			pickle.dump(ts, f)
 		print('pickle saved')