Fix 42-minute test-3.12 CI runs with coverage sysmon core

CPython 3.12's settrace-based tracing is pathologically slow under
coverage (CPython gh-107674, coverage.py #1665), making pytest --cov
take ~42 minutes on the 3.12 matrix job vs ~40 seconds elsewhere.
Export COVERAGE_CORE=sysmon on Python 3.12+ so coverage uses the
sys.monitoring core; guarded because sysmon does not exist on <=3.11.
Verified locally: full suite on 3.12 with coverage drops from ~42 min
to 3.6 s with identical line-coverage results.

Author: bobbypaton

.circleci/config.yml

@@ -16,7 +16,14 @@ jobs:
             pip install -e ".[dev]"
       - run:
           name: Run tests with coverage
-          command: pytest --cov=pyqrc --cov-report=xml -v
+          command: |
+            # CPython 3.12's settrace is pathologically slow under coverage
+            # (gh-107674); the sys.monitoring core avoids it but only exists
+            # on 3.12+.
+            if python -c 'import sys; sys.exit(sys.version_info < (3, 12))'; then
+              export COVERAGE_CORE=sysmon
+            fi
+            pytest --cov=pyqrc --cov-report=xml -v
       - run:
           name: Upload coverage to Codecov (non-blocking)
           command: |