Add code coverage

Author: yeger00

.github/workflows/test-pkg.yml

@@ -27,4 +27,10 @@ jobs:
         run: make lint
 
       - name: test
-        run: make test
+        run: PYTEST_FLAGS='--cov-report json --cov-report term --cov=pylspclient --cov-branch' make test
+
+      - name: upload coverage
+        uses: actions/upload-artifact@v2
+        with:
+          name: coverage
+          path: ./htmlcov