python-cffi: switch to notmuch_database_{open,create}_with_config

Since release 0.32, libnotmuch provides searching for database and
configuration paths. This commit changes the python module notmuch2 to
use those facilities.

This fixes the bug reported in [1], along with a couple of the
deprecation warnings in the python bindings.

Database.default_path is deprecated, since it no longer faithfully
reflects what libnotmuch is doing, and it is also no longer used in
the bindings themselves.

This commit choose the default of config=CONFIG.EMPTY (equivalent to
passing "" to notmuch_database_open_with_config).  This makes the
change upward compatible API-wise (at least as far as the test suite
verifies), but changing the default to CONFIG.SEARCH would probably be
more convenient for bindings users.

[1]: id:87h7d4wp6b.fsf@tethera.net