CI: Set COVERAGE_CORE to speed up tests

Sets `COVERAGE_CORE` to "sysmon" to speed up testing on Python 3.12.
The "normal" setting is very slow on Python 3.12 when also checking the
line coverage.

.github/workflows/ci_build_and_test.yml

@@ -64,6 +64,8 @@ jobs:
         run: |
           py.test -v --doctest-glob='*.md' --cov src
           coveralls --build-root=. --include=src --dump=cpp-coveralls.json
+        env:
+          COVERAGE_CORE: sysmon
       - name: Convert coverage
         run: python -m coverage xml
       - name: Upload coverage report