[PATCH] python: Fix database.add_message

[Gaute Hope]

David Bremner writes on september 12, 2017 3:56:
> Dylan Baker <dylan at pnwbakers.com> writes:
> 
>> b10ce6bc23002d48916b1b2f375480e7540e3164 introduced a new index_file
>> method as a replacement for add_message. It helpfully made add_message
>> an alias for index_file, unfortunately it got the signature wrong by
>> explicitly passing self as an argument.
>>
>> cc: Daniel Kahn Gillmor <dkg at fifthhorseman.net>
> 
> Thanks. We have another version that I was about to commit
> 
> https://www.mail-archive.com/notmuch@notmuchmail.org/msg45038.html
> 
> The only difference is that Gaute (more or less by request) added a
> deprecation warning.  I don't really know how warnings in python work,
> does that sound sensible?

Yup, also notice the missing `return`.

Regards, Gaute