Add configuration for coverage

.github/workflows/python-coverage.yml

@@ -0,0 +1,43 @@
+name: python coverage
+
+on:
+  push:
+    branches:
+      - master
+  pull_request: {}
+
+jobs:
+  coverage:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v3
+
+    - name: Setup Python
+      uses: actions/setup-python@v4
+      with:
+        python-version: '3.10'
+        architecture: x64
+
+    - name: Install dependencies
+      # python_tests requires optuna>=3.0.0 since it imports FloatDistribution
+      run: |
+        python -m pip install --progress-bar off --upgrade pip setuptools
+        pip install --progress-bar off "optuna>=3.0.0"
+        pip install --progress-bar off .
+        echo 'import coverage; coverage.process_startup()' > sitecustomize.py
+
+    - name: Tests
+      env:
+        PYTHONPATH: .  # To invoke sitecutomize.py
+        COVERAGE_PROCESS_START: .coveragerc  # https://coverage.readthedocs.io/en/6.4.1/subprocess.html
+        COVERAGE_COVERAGE: yes  # https://github.com/nedbat/coveragepy/blob/65bf33fc03209ffb01bbbc0d900017614645ee7a/coverage/control.py#L255-L261
+      run: |
+        coverage run --source=optuna -m pytest tests -m "not skip_coverage and not slow"
+        coverage combine
+        coverage xml
+
+    - name: Upload coverage to Codecov
+      uses: codecov/codecov-action@v3
+      with:
+        file: ./coverage.xml