rebase with upstream release 0.34.3
    
    bz #2030690
    
    Library
    -------
    
    Do not crash when presented with a .notmuch directory without a
    xapian/ subdirectory.
    
    Python Bindings (notmuch2)
    --------------------------
    
    Database constructor now searches for configuration by default. Pass
    `config=Database.CONFIG.EMPTY` to disable.
    
    The `Message.replies()` method now returns OwnedMessage objects, to
    prevent certain memory de-allocation errors.
    
    Fix for importing `notmuch2` module when building bindings
    documentation.