Add __aiter__ and __anext__ methods for asyncio.StreamReader

[dlax]

The following code produces an error in mypy:

import asyncio
from asyncio.subprocess import PIPE

async def main() -> None:
    proc = await asyncio.create_subprocess_shell("ls -l", stdout=PIPE)
    assert proc.stdout is not None
    async for line in proc.stdout:
        print(line.decode())
    await proc.wait()

asyncio.run(main())

$ mypy --strict file.py
file.py:8: error: "StreamReader" has no attribute "__aiter__" (not async iterable)

This commits fixes this by adding __aiter__/__anext__ methods that are
needed for async iterator protocol.