Retire wait_for_psce: deprecation shim over asyncio.timeout, floor py3.11

[majkelx]

Part of the retire wait_for_psce epic: araucaria-project/ocabox-common#15 (full rationale there). Summary: wait_for_psce works around an asyncio.wait_for bug fixed in CPython long ago (3.12 reimplemented wait_for on asyncio.timeout()); the workaround's shield(wait_for(task)) sandwich creates orphanable middle tasks (Task exception was never retrieved, see ocabox-server#46) and can replace CancelledError with the task's real exception on the cancel path.

Replacement pattern

# before:
result = await wait_for_psce(coro(...), timeout=t)

# after (py ≥ 3.11):
async with asyncio.timeout(t):
    result = await coro(...)

Rules:

• TimeoutError is asyncio.TimeoutError since 3.11 — do NOT touch existing exception handlers.
• Negative/zero timeouts behave the same (immediate TimeoutError).
• When the argument is an already-created Task/Future (not a coroutine), the pattern is async with asyncio.timeout(t): await fut — works, but double-check the site did not rely on the extra task wrapper.
• Remove any # do not use asyncio.wait_for(), use wait_for_psce() warning comments near the call sites.
• Behavior must stay identical for callers: same result, same TimeoutError on timeout, CancelledError (not the inner exception) when the caller is cancelled.

Scope (this repo — PUBLIC API, deprecate instead of delete)

wait_for_psce is exported from serverish.base — external users may depend on it, so this repo gets a deprecation shim, not deletion:

• Bump python = "^3.10" → "^3.11" in pyproject.toml.
• Migrate internal call sites:
  • serverish/messenger/msg_reader.py — 1 site (wait_for_psce(self._msg_processed.wait(), timeout=to)).
  • serverish/base/task_manager.py — 1 site (wait_for_psce(task.task, timeout) — NOTE: the argument is an existing Task; use async with asyncio.timeout(timeout): await task.task and double-check the surrounding cancel/cleanup logic still holds).
• Replace the body of serverish/base/asyncio_util_functions.py::wait_for_psce with a thin shim:

  async def wait_for_psce(fut, timeout):
      """Deprecated: use asyncio.timeout(). Kept for API compatibility."""
      warnings.warn("wait_for_psce is deprecated; use asyncio.timeout()",
                    DeprecationWarning, stacklevel=2)
      async with asyncio.timeout(timeout):
          return await fut

  Keep the serverish.base export. Removal is a future major-version decision.
• Version bump REQUIRED in this PR: 2.3.0 → 2.4.0 in pyproject.toml (this repo keeps no CHANGELOG — skip that part).

Acceptance

• All listed call sites migrated; the local wait_for_psce copy deleted; no remaining references (grep -r wait_for_psce).
• Python floor bumped as specified; version bump + compact CHANGELOG entry.
• Cancellation-semantics tests (add if missing): (1) result delivery, (2) TimeoutError on timeout, (3) caller cancelled → CancelledError propagates and the awaited work is cancelled. Model them on ocabox-server/test/utils/test_wait_for_psce.py (post-#47), adapted to the new pattern.
• Existing test suite green.

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