Python 3.15 issues

[frenzymadness]

When building jupyter_server with Python 3.15 alpha 7, the tests fail because:

      File "/usr/lib64/python3.15/pty.py", line 66, in fork
        pid, fd = os.forkpty()
                  ~~~~~~~~~~^^
    DeprecationWarning: This process (pid=1305) is multi-threaded, use of forkpty() may lead to deadlocks in the child.

The filterwarnings setting here makes this an error. I'm not sure this is something worth reporting to ptyprocess library.