Address "Do not use mutable data structures for argument defaults" warning

[palewire]

flake8-bugbear in #318 is raising the following warnings. We should address them.

```bash
./warn/platforms/job_center/cache.py:27:40: B006 Do not use mutable data structures for argument defaults.  They are created during function definition time. All calls to the function reuse this one instance of that data structure, persisting changes between them.
./warn/platforms/job_center/site.py:101:37: B006 Do not use mutable data structures for argument defaults.  They are created during function definition time. All calls to the function reuse this one instance of that data structure, persisting changes between them.
./warn/platforms/job_center/site.py:121:27: B006 Do not use mutable data structures for argument defaults.  They are created during function definition time. All calls to the function reuse this one instance of that data structure, persisting changes between them.
./warn/platforms/job_center/site.py:227:58: B006 Do not use mutable data structures for argument defaults.  They are created during function definition time. All calls to the function reuse this one instance of that data structure, persisting changes between them.
./warn/scrapers/fl.py:188:34: B006 Do not use mutable data structures for argument defaults.  They are created during function definition time. All calls to the function reuse this one instance of that data structure, persisting changes between them.

[palewire]

This was fixed by #323