[workflows] add proper codecov action

Author: pawelmhm

.github/workflows/main.yml

@@ -64,5 +64,5 @@ jobs:
         pip install -U tox
         tox
 
-    - name: Upload coverage report
-      run: bash <(curl -s https://codecov.io/bash)
+    - name: Upload to codecov
+      uses: codecov/codecov-action@v2

tox.ini

@@ -12,4 +12,4 @@ deps =
     -r{toxinidir}/requirements-test.txt
     msgpack: -r{toxinidir}/requirements.txt
     pypy-msgpack: -r{toxinidir}/requirements-pypy.txt
-commands = py.test --cov=scrapinghub --cov-report=
+commands = py.test --cov=scrapinghub --cov-report=xml