Cursor.copy() made into a context manager

Cursor.copy() made into a context manager

What was before was a factory function, however that forced to have a
pattern like:

    async with (await cursor.copy()) as copy

Now instead what should be used is:

    async with cursor.copy() as copy

With this change the user pretty much is never exposed anymore to a Copy
object in a non-entered state. This is actually useful because it
reduces the surface of the API: now for instance Copy.finis() can become
a private method.