Add basic migration system

[abulte]

Pending #29, this allows us to write simple SQL migrations.

The library is unfortunately not published on pypi, but code is clean and simple. This should only be temporary anyway.

I made a small hack with the _LATEST_REVISION file in order to be able to upgrade to the latest revision (sic) without knowing its name.

[abulte]

@maudetes asyncpg-trek is now on pypi! And I'm an official contributor ;-)

[adriangb]

Very cool! I’m curious how you folks feel about the whole “graph of versions” vs linear migrations (hence the {YYYYMMDD}_rev{from}_up_rev{to}.sql syntax). I quite like that the foundation of asyncpg-trek is devoid of restrictive linear representations of migrations, but maybe it makes sense to add a hardcoded pointer to the latest version and a CLI like you have here?

[maudetes]

Nice! I find the usage and the _LATEST_REVISION hack simple enough!

I added a very blocking comment, though :p

[abulte]

@adriangb I have to admit I've never used a migration system with such a feature, so I'm not sure what use case it could apply to. Maybe ideas will come over time? We're not using it for complicated stuff right now. I don't known if it's linked but your library was by far the simplest (maintained) I've seen. I only had to resort to the _LATEST_REVISION file hack to make it work for us, since I did not want every dev and ops from the team to be aware of this latest revision.