snapshot: lint list-last-n-good-dumps.py

Fix a few trivial errors reported by flake8:

list-last-n-good-dumps.py:

Change-Id: If36c38c4fd7838386dbf7217c7d9139ae17ad03f
3:1: F401 'subprocess' imported but unused
171:42: E226 missing whitespace around arithmetic operator
412:47: E226 missing whitespace around arithmetic operator
413:36: E226 missing whitespace around arithmetic operator
416:71: E226 missing whitespace around arithmetic operator