fix: restore Python 3.9/3.10 support broken in the v0.7.0 CI matrix

Two independent failures, both confirmed against the tests workflow on
246ec5c (Rust tests green, 3.9 and 3.10 red):

* 3.9 - `flush_usage` used a bare PEP 604 `str | None` in its signature.
  Annotations are evaluated at definition time, and `type.__or__` only
  exists from 3.10, so importing the package raised
  `TypeError: unsupported operand type(s) for |: 'type' and 'NoneType'`
  at __init__.py:175. Quote the annotation so it is never evaluated;
  `_Optional` is not imported until line 499. FA102 is removed from the
  ruff ignore list so this cannot regress silently again - it is a
  correctness guard for `requires-python = ">=3.9"`, not a style rule.

* 3.10 - three packaging tests imported `tomllib` unconditionally, which
  only entered the stdlib in 3.11, so the 3.9/3.10 legs died at
  collection. Fall back to `tomli` the same way test_api_compat.py
  already does, and declare `tomli` for python_version < 3.11 so the
  tests actually run on those legs instead of erroring or skipping.

Both were latent: earlier runs failed at the build or lint step and
never reached them.

Author: qectorlab

pyproject.toml

@@ -59,6 +59,10 @@ dev = [
     # A9-01: `tests.yml` invokes `pytest --timeout=120`. Without this plugin that
     # is an unrecognized argument and the whole job dies at collection.
     "pytest-timeout>=2.1.0",
+    # Four packaging tests parse pyproject.toml with `tomllib`, which only
+    # entered the stdlib in 3.11. Without this the 3.9/3.10 matrix legs die at
+    # collection with ModuleNotFoundError rather than running the tests.
+    "tomli>=2.0; python_version<'3.11'",
     "hypothesis>=6.0.4",  # clears 1 Safety advisory; 6.0.4 still allows py3.6+
     "fastapi>=0.110.0",
     "uvicorn>=0.29.0",
@@ -195,8 +199,12 @@ docstring-code-format = true
 # E702: semicolons used intentionally for chained one-liners in plotting helpers.
 # E731: lambda assignments used for compact callback tables in a few helpers.
 # E741: single-letter variable names (l, I, O) are domain-conventional in QEC math.
-# S110, BLE001, FA102, C414, C401, RUF*, ISC004, UP045, B017, PLW1510, SIM118: style & optional-dep guards.
-ignore = ["E702", "E731", "E741", "S110", "BLE001", "FA102", "FA100", "C414", "C401", "RUF012", "RUF023", "RUF034", "RUF046", "RUF059", "RUF100", "ISC004", "UP045", "B017", "PLW1510", "SIM118"]
+# S110, BLE001, C414, C401, RUF*, ISC004, UP045, B017, PLW1510, SIM118: style & optional-dep guards.
+#
+# FA102 is deliberately NOT ignored: `requires-python = ">=3.9"`, and a bare
+# PEP 604 `X | Y` in a signature is evaluated at import time, so on 3.9 it
+# raises TypeError and makes the whole package unimportable. Keep it enforcing.
+ignore = ["E702", "E731", "E741", "S110", "BLE001", "FA100", "C414", "C401", "RUF012", "RUF023", "RUF034", "RUF046", "RUF059", "RUF100", "ISC004", "UP045", "B017", "PLW1510", "SIM118"]
 
 [tool.ruff.lint.per-file-ignores]
 # __init__: late imports (E402) are intentional (runtime optional deps).

python/qector_decoder_v3/__init__.py

@@ -172,7 +172,7 @@ def get_accumulated_shots() -> int:
     return 0
 
 
-def flush_usage(customer_id: str | None = None, api_key: str | None = None) -> dict:
+def flush_usage(customer_id: "str | None" = None, api_key: "str | None" = None) -> dict:
     """Flush accumulated decode shots to the Stripe Billing Meter Events API.
 
     ``customer_id`` falls back to ``QECTOR_STRIPE_CUSTOMER_ID`` and ``api_key``

python/tests/test_license_included.py

@@ -10,7 +10,10 @@
 
 import os
 
-import tomllib
+try:
+    import tomllib
+except ModuleNotFoundError:  # Python < 3.11 - tomllib entered the stdlib in 3.11
+    import tomli as tomllib
 
 
 def _repo_root():

python/tests/test_optional_extras.py

@@ -9,7 +9,11 @@
 import os
 
 import pytest
-import tomllib
+
+try:
+    import tomllib
+except ModuleNotFoundError:  # Python < 3.11 - tomllib entered the stdlib in 3.11
+    import tomli as tomllib
 
 
 def _repo_root():

python/tests/test_version_consistency.py

@@ -14,7 +14,11 @@
 import os
 
 import qector_decoder_v3
-import tomllib
+
+try:
+    import tomllib
+except ModuleNotFoundError:  # Python < 3.11 - tomllib entered the stdlib in 3.11
+    import tomli as tomllib
 
 
 def _repo_root():