Support subclassing Connection

[PikalaxALT]

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Description

Add cls kwarg to core.connect that is a subclass or Connection (or a Callable[[Callable[[Any], sqlite3.Connection], int], Connection]) (default: Connection)

Example usage:

class MyConnection(aiosqlite.Connection):
    ...

async def main():
    conn = await aiosqlite.connect(':memory:', cls=MyConnection)
    # use methods and attrs of MyConnection

Current workaround: connect must be redefined to explicitly call your custom async connection factory.

[amyreese]

Sounds good. Can you add a simple test case covering this?