Require ezmsg 3.10.0b5; describe the witness across a process hop

[cboulay]

• Bumps ezmsg[axisarray] from >=3.10.0b3 to >=3.10.0b5.
• Updates the hash-witness comments in stateful.py for what b5 changes:
  • The fingerprint is computed when an axis is pickled (CoordinateAxis.fingerprint: compute when pickling, widen to 64 bits ezmsg#274), so it always arrives precomputed past a process hop.
  • With axis elision (Elide coordinate axes a receiving channel already holds ezmsg#276), the receiving channel hands every message the same coordinate axis object. The witness's weak-reference identity check therefore usually hits across a hop too, not only in-process.
  • The value comparison and the weak reference (Hold the hash witness's axes by weak reference #18) still matter whenever elision is off: an older peer, EZMSG_DISABLE_AXIS_ELISION, or an axis without a fingerprint.

No code changes. Checked against ezmsg 3.10.0b5 from PyPI:

• Test suite: 242 passed. The one failure, test_producer_handles_missing_graph_server_gracefully, is pre-existing: it fails whenever a GraphServer is already listening on the default port.
• End to end over SHM: elision on, one shared channel axis past the hop, the witness's weak reference alive and identical to it, and a single reset across 5 messages.