Add proxy for autocommit property on Connection

[jonathan-d-zhang]

Description

Add a proxy property for the autocommit property on sqlite3.Connection objects.

Use Case

I want autocommit=False as suggested by the sqlite3 docs (see docs). I also want to set PRAGMA journal_mode = WAL. The issue is that I can't change the journal_mode when in a transaction, which is always the case when autocommit=False. So the solution is to set the autocommit property after setting the journal_mode. This works fine in sqlite3, but aiosqlite doesn't have the property.

I can work around this by simply reaching into the aiosqlite.Connection object and accessing the _conn attribute, but this is clearly not ideal.