fix: resolve potential race condition in JSONFileCache

[msw-kialo]

The JSONFileCache.__setitems__ might raise a FileExistsError if it is run in parallel by two threads / processes:

1. Both processes discover the working dir hasn't been created yet.
2. The first process creates the working dir.
3. The second process tries to create the working dir, too and fails as it already exists.

Accept this potential race — exist_ok=True will prevent os.makedirs to raise an exception.

The os.path.isdir condition is technically no longer needed but kept keeping the change small. Furthermore, it should be slightly faster as the cache should exist most of the time.

The exist_ok parameter is supported since Python 3.2 / fixed in 3.4.1 and therefore available in all needed python versions.