ci: switch to Coveralls' first-party GitHub Action

Dropped `coveralls` dev-requirement in favor of using the `coverage`
package directly, and upgraded it to the latest release series (7.x).

The `line_bits` table in .coverage (SQLite DB of results, generated by
`coverage`) isn't populated. coverallsapp/coverage-reporter tries an SQL
query against the database based on this empty table, and fails because
there aren't any coverage results to upload.

We work around this by having `coverage lcov` generate a file format
that the reporter can understand.

.github/workflows/ci.yml

@@ -41,12 +41,13 @@ jobs:
         run: make lint
       - name: Run pytest
         run: make test_norecord
+      - name: Generate lcov coverage report
+        run: coverage lcov
       - name: Upload coverage data to coveralls.io
-        run: coveralls --service=github
-        env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-          COVERALLS_FLAG_NAME: ${{ matrix.python-version }}
-          COVERALLS_PARALLEL: true
+        uses: coverallsapp/github-action@v2
+        with:
+          flag-name: ${{ matrix.python-version }}
+          parallel: true
 
   coveralls:
     if: ${{ github.repository_owner == 'sopel-irc' }}
@@ -55,11 +56,9 @@ jobs:
     name: Finalize job on coveralls.io
     needs: tests
     runs-on: ubuntu-latest
-    container: python:3-slim
     steps:
     - name: Notify coveralls.io
-      run: |
-        pip install --upgrade coveralls
-        coveralls --finish
-      env:
-        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      uses: coverallsapp/github-action@v2
+      with:
+        parallel-finished: true
+        carryforward: all

dev-requirements.txt

@@ -1,5 +1,5 @@
 # These are requirements to develop Sopel itself.
-coveralls>=2.0
+coverage~=7.0
 flake8>=5
 flake8-coding
 flake8-future-import