coveralls for 3.x only, add paralle unique build key

disable upload to coveralls for 2.7. I need some way under python 2.x
to generate coverage lcov syle files. Otherwise I lose coverage for
the 2.x code paths.

Call 'coverage lcov' only in 3.x pytest codepath not 2.x.

Add unique tag for each parallel upload for coveralls.
Also set lcov file name to what coverage lcov outputs.

Author: rouilj

.github/workflows/ci-test.yml

@@ -193,6 +193,7 @@ jobs:
               -W "ignore:unclosed:ResourceWarning:roundup.roundup.demo" \
               -W "ignore:unclosed file:ResourceWarning:enum" \
               -v  test/ --cov=roundup
+            codecov lcov
           else
             # python2 case
             pytest -v -r a test/ --cov=roundup
@@ -206,9 +207,13 @@ jobs:
           token: ${{ secrets.CODECOV_TOKEN }}
 
       - name: Upload coverage to Coveralls
+        # python 2.7 version of codecov can't produce lcov files.
+        if: matrix.python-version != '2.7'
         uses: coverallsapp/github-action@master
         with:
           github-token: ${{ secrets.GITHUB_TOKEN }}
+          path-to-lcov: coverage.lcov
+          parallel: run-{{ matrix.python-version }}-{{ matrix.os }}
 
       - name: test build_doc
         run: |