This is the more modern way to manage concurrency with SQLite

In our case, it means concurrent mypy runs using the cache will wait for
each other, rather than fail

SQLite also claims this is faster, but I haven't yet done a good profile
(If you are profiling this, note that WAL is a persistent setting, so
you will want to delete the cache)

Finally, I also explicitly close the connection in main. This is
relevant to this change, because it forces checkpointing of the WAL,
which reduces disk space and means the cache.db remains a single
self-contained file in regular use