Adjust how we build TOXENV in CI

run tests for only 1 Python + Django combination

Author: atodorov

.github/workflows/testing.yml

@@ -39,7 +39,7 @@ jobs:
       fail-fast: false
       matrix:
         python-version: [3.6, 3.7, 3.8, 3.9]
-        django-version: [master, 3.2, 3.1, 3.0, 2.0, 1.11]
+        django-version: [-master, 3.2, 3.1, 3.0, 2.0, 1.11]
 
     steps:
       - uses: actions/checkout@v2
@@ -55,6 +55,7 @@ jobs:
 
             export DJANGO=${{ matrix.django-version }}
             export PYTHON=${{ matrix.python-version }}
+            export TOXENV=$(echo py${{ matrix.python-version }}-django${{ matrix.django-version }} | tr -d .)
             tox
 
       - name: Coveralls