Open py-version range + harness gate for py3.14 backends (#379)

Prep for a future sub-interpreter (PEP 734
`concurrent.interpreters`) spawn backend per issue
test-harness error-gating; the backend itself comes
later.

Deats,
- bump `pyproject.toml` `requires-python` to
  `>=3.12, <3.15` and list the `3.14` classifier —
  the new stdlib `concurrent.interpreters` module
  only ships on 3.14
- `_testing.pytest.pytest_configure` wraps
  `try_set_start_method()` in a `pytest.UsageError`
  handler so an unsupported `--spawn-backend` on the
  running py-version prints a clean banner instead
  of a traceback

(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

(cherry picked from commit d318f1f8f4)
(factored: kept only the pyproject + `_testing/pytest.py` parts of
 "Add `'subint'` spawn backend scaffold (#379)"; dropped
 tractor/spawn/_spawn.py + tractor/spawn/_subint.py)

tractor/_testing/pytest.py

@@ -227,7 +227,13 @@ def pytest_addoption(
 def pytest_configure(config):
     backend = config.option.spawn_backend
     from tractor.spawn._spawn import try_set_start_method
+    try:
         try_set_start_method(backend)
+    except RuntimeError as err:
+        # e.g. `--spawn-backend=subint` on Python < 3.14 — turn the
+        # runtime gate error into a clean pytest usage error so the
+        # suite exits with a helpful banner instead of a traceback.
+        raise pytest.UsageError(str(err)) from err
 
     # register custom marks to avoid warnings see,
     # https://docs.pytest.org/en/stable/how-to/writing_plugins.html#registering-custom-markers