database.py: stream of json objects forward compat (#43598)

In the future we may transform the database from a single JSON object to
a stream of JSON objects.

This paves the way for constant time writes and constant time rereads
when only O(1) changes are made. Currently both are linear time.

This commit gives just enough forward compat for Spack to produce a
friendly error when we would move to a stream of json objects, and a db
would look like this:

```json
{"database": {"version": "<something newer>"}}
```

0 files changed, 0 insertions, 0 deletions