python bindings memory management

[Gaute Hope]

Hi,

when writing this small script [0][1] using the notmuch python bindings I 
had to manually delete the python objects in the right order. Otherwise 
it segfaults when exiting, presumably because memory dependencies / 
cleanup isn't done correctly when the Python GC gets going. Essentially 
I have to delete the objects in the order of their dependency:

```
  db = notmuch.Database()
  q = db.create_query ("some query")
  q.set_sort (notmuch.Query.SORT.OLDEST_FIRST) # probably not necessary
  msgs = q.search_messages ()


  for m in msgs:
    pass

  # remove these to get segfault
  del m
  del msgs
  del q

```

Regards, Gaute

[0] https://gist.github.com/gauteh/de546cb143ebae4a489a482a05ab4beb
[1] id:1507537566.y6qjbg4xoq.astroid at strange.none