Split library into two parts: DB-API 2.0 compatible and Asyncio style

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I am very interested in using this library since type support in `psycopg2` is awful to work with. However, some projects still use Python 2.7, and will for another 4 years. Would it be possible to split `asyncpg` into two parts, one with an asynchronous API, usable with Python 3.5 and above, and one synchronous API that can be compatible across both 2.7 and 3.5 and above?

My proposal is a DB-API 2.0 ([PEP 249](https://www.python.org/dev/peps/pep-0249/)) compatible API that can be utilized by the `asyncio` part of the library. The benefits is that applications that do not need asynchronous calls can use the same underlying library as those who do. With this code that doesn't rely on co-routines can use the type support provided by `asyncpg`.

I haven't used this project yet, or dived into its internals, and don't know whether it is feasible at all to do this. I am just venting my ideas.

La issue non indica alcun file di implementazione né test. Inizia esaminando l'API asyncio esistente di asyncpg e confrontando l'interfaccia sincrona DB-API 2.0 proposta con PEP 249 e la compatibilità con Python 2.7. Poiché si tratta di una proposta architetturale esplorativa, i maintainer dovrebbero prima definire la fattibilità, l'ambito e uno stato di completamento misurabile.

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