Drop AbstractAsyncContextManager for proper type hint to BaseSession

[Kludex]

You can see that session assumes ClientSession type now.

from mcp.client.session import ClientSession


async def main():
    async with ClientSession() as session:
        await session.initialize()

The real problem is that AbstractAsyncContextManager is a generic, so it expects some type as an argument. We can't really add BaseClient as that type, because the return type from the ClientSession __aexit__ is not BaseClient, it's actually ClientSession.

[dsp-ant]

Isn't this in typing as well?

[dsp-ant]

my bad, it's in typing in 3.11 not in 3.10