Add codecov support

Author: sliverc

.github/workflows/tests.yml

@@ -12,6 +12,10 @@ jobs:
         python-version: ["3.6", "3.7", "3.8", "3.9"]
         django: ["2.2", "3.0", "3.1"]
         django-rest-framework: ["3.12", "master"]
+    env:
+      PYTHON: ${{ matrix.python-version }}
+      DJANGO: ${{ matrix.django }}
+      DJANGO_REST_FRAMEWORK: ${{ matrix.django-rest-framework }}
     steps:
       - uses: actions/checkout@v2
       - name: Set up Python ${{ matrix.python-version }}
@@ -24,9 +28,10 @@ jobs:
           pip install tox tox-gh-actions
       - name: Test with tox
         run: tox
-        env:
-          DJANGO: ${{ matrix.django }}
-          DJANGO_REST_FRAMEWORK: ${{ matrix.django-rest-framework }}
+      - name: Upload coverage report
+        uses: codecov/codecov-action@v1
+        with:
+          env_vars: PYTHON,DJANGO,DJANGO_REST_FRAMEWORK
   check:
     name: Run check
     runs-on: ubuntu-latest