Simple fix for notmuch database not in sync

I first intended to fix https://github.com/enkore/i3pystatus/issues/121 by capturing stderr from the C library but it introduces too much complexity. Instead this patch just opens and closes the database for each request. I don't believe it has much overhead (just opening a file) though I have no benchmark. I believe it's possible to open the db, <db modified by an outer program>, then having the notmuch request fail (because of an outdated db) but with a much lower probability. Anyway it would be refreshed in the next interval. Hope notmuch team fixes this though.
Don't you need to close `db` somewhere?