Previously pbspy could only submit jobs when running directly on a
supercomputer login node.  This adds a client/server architecture
so jobs can be submitted from any machine with SSH access.

src/pbspy/__init__.py has been split into discrete modules:

  _backend.py        - Backend Abstract Base Class
  _local_backend.py  - LocalBackend: direct qsub/qstat calls (default)
  _ssh_backend.py    - SSHBackend: talks to the daemon over SSH
  _pbs_core.py       - Core PBS operations extracted from __init__.py
  _protocol.py       - Length-prefixed pickle framing (send_frame /
                        recv_frame) and request/response dataclasses

A new top-level package pbspy_server contains:

  server.py   - TCP daemon: accepts connections, dispatches requests,
                polls qstat in a background thread, streams status
                updates to waiting clients, shuts down automatically
                when no jobs are pending and no clients are connected
  proxy.py    - SSH proxy mode: acquires a file lock, starts the daemon
                if needed, then splices stdin/stdout with the daemon
                socket so the remote client communicates transparently
  __main__.py - CLI entry point (--daemon / --proxy / --status)

pyproject.toml adds the `pbspy-server` console script.

`pbspy-server` binds a TCP port on all interfaces so that proxy
processes on other login nodes can reach it.  To prevent unauthorized
access (pickle deserialization is remote code exec), every connection
must authenticate with a shared secret token:
  - On startup the server generates secrets.token_hex(32) and writes the
    addr file on the login-node as JSON: {"host": ..., "port": ..., "token": ...}
  - Clients send AuthRequest(token=...) as the first frame; the server
    responds with AuthOkResponse or ErrorResponse and closes the connection
  - The proxy reads the token from the JSON addr file and authenticates
    using it
  - pbspy-server --status and _daemon_alive() also authenticate before
    sending PingRequest

  JobDescription.submit() now accepts an optional `backend` parameter
  (defaults to LocalBackend).  Pass an SSHBackend instance to submit to
  a remote supercomputer:
  ```python
      backend = SSHBackend("user@gadi.nci.org.au")
      job = JobDescription(...).add_command([...]).submit(backend=backend)
  ```
  Backend, LocalBackend, and SSHBackend are exported from pbspy.__all__.

Added tests to new code:
  tests/test_protocol.py
  tests/test_server.py
  tests/test_default.py
Updated pre-commit:
  .pre-commit-config.yaml: added pytest to mypy's additional_dependencies so
  the type stubs for pytest decorators are available during the hook run.