ci/coveralls: replace github-action with python package

https://github.com/coverallsapp/github-action/issues/4#issuecomment-529399410
https://github.com/aodj/icelandreview/commit/9bc1a6c8753d583905498f76cea4d5bc3506ed89#diff-fd3c00174dcaf7b8a36765f1a3de6791

.github/workflows/python.yml

@@ -39,7 +39,7 @@ jobs:
     - run: pipenv run pylint --disable=missing-requirement --disable=parse-error tests/*
     - run: pipenv run pytest --cov=ical2vdir --cov-report=term-missing --cov-fail-under=100
     - run: pipenv run black --check .
-    - uses: coverallsapp/github-action@v1.0.1
-      with:
-        path-to-lcov: ./.coverage
-        github-token: ${{ secrets.GITHUB_TOKEN }}
+    - run: pip install 'coveralls<2'
+    - run: coveralls
+      env:
+        COVERALLS_REPO_TOKEN: ${{ secrets.GITHUB_TOKEN }}