micropython/streampair: Package to create bi-directional linked stream objects.

[andrewleech]

Returns two bi-directional linked stream objects where writes to one can be read from the other and vice/versa.

This can be used somewhat similarly to a socket.socketpair in python, like a pipe of data that can be used to connect stream consumers (eg. asyncio.StreamWriter, mock Uart)

[andrewleech]

The RingIO module has been merged in micropython so this should be updated to use it

[andrewleech]

Updated this PR with the following changes:

API Updates:

• Replace deprecated ringbuffer with RingIO name (they're the same underlying C type, but RingIO is the documented API name)
• Remove unused collections.deque import

New Tests:
Added two tests that demonstrate and verify the ioctl protocol implementation works correctly with select.poll() and asyncio:

1. test_select_poll_compatibility() - Verifies that StreamPair objects can be registered with select.poll() and correctly report POLLIN events when data is available

2. test_streamreader_direct_usage() - Demonstrates that StreamPair works directly with asyncio.StreamReader without needing any wrapper or special handling

Technical Detail:

The existing ioctl implementation already correctly returns -1 (_MP_STREAM_ERROR) for unsupported operations.
This is the protocol requirement that enables select.poll() to work with pure Python stream objects.

Since asyncio.StreamReader is built on top of select.poll(), this means StreamPair (and any Python object with proper ioctl implementation) works seamlessly with async I/O.

These tests serve as documentation that pure Python stream objects don't require C-level integration to work with micropython's polling and async infrastructure - they just need to implement the ioctl protocol correctly.