coverage: add new job to run after build

Build job:
- add upload artifact step after each test in the matrix.
- update coveralls step to upload partial data.

Converage job (new):
- complete the coveralls in the coverage job.
- add step to download all uploaded artifacts
- add step to combine coverage data, create report and fail if not
  enough
- add step to upload html report if coverage is not enough

.github/workflows/tests.yml

@@ -29,12 +29,10 @@ jobs:
         - ["3.10",  "py310"]
         - ["3.11.0-rc.2",  "py311"]
         - ["3.9",   "docs"]
-        - ["3.9",   "coverage"]
         - ["3.9",   "py39-datetime"]
         exclude:
           - { os: windows, config: ["3.9",   "lint"] }
           - { os: windows, config: ["3.9",   "docs"] }
-          - { os: windows, config: ["3.9",   "coverage"] }
 
     runs-on: ${{ matrix.os }}-latest
     if: github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name != github.event.pull_request.base.repo.full_name
@@ -58,13 +56,50 @@ jobs:
         python -m pip install --upgrade pip
         pip install tox
     - name: Test
-      if: matrix.config[1] != 'coverage'
       run: tox -e ${{ matrix.config[1] }}
-    - name: Coverage
-      if: matrix.config[1] == 'coverage'
-      run: |
-        tox -e py36,py37,py38,py39,py39-datetime,py310,py311,coverage
-        pip install coveralls
-        coveralls --service=github
-      env:
-        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+
+    - name: Upload coverage artifact
+      uses: actions/upload-artifact@v2
+      if: matrix.os != 'windows'
+      with:
+        name: coverage-data
+        path: .coverage.${{ matrix.config[1] }}
+        if-no-files-found: ignore
+
+  coverage:
+    name: Combined coverage
+    runs-on: ubuntu-latest
+    needs: build
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-python@v2
+        with:
+          # Use latest, so it understands all syntax.
+          python-version: "3.10"
+      - run: python -m pip install --upgrade coveralls coverage[toml]
+
+      - name: Download coverage data.
+        uses: actions/download-artifact@v2
+        with:
+          name: coverage-data
+
+      - name: Indicate completion to coveralls.io
+        run: |
+          python -m coverage combine
+          coveralls --service=github
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Coverage report and fail if it's <100%.
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          python -m coverage html --skip-covered --skip-empty
+          python -m coverage report --fail-under=100
+
+      - name: Upload HTML report if check failed.
+        uses: actions/upload-artifact@v2
+        with:
+          name: html-report
+          path: htmlcov
+        if: ${{ failure() }}