Fix CI config matrix

Author: kislyuk

.github/workflows/ci.yml

@@ -3,11 +3,12 @@ name: Test suite
 on: [push]
 
 jobs:
-  run_unit_tests:
-    runs-on: [ubuntu-18.04, ubuntu-20.04, macos-10.15]
+  unit_tests:
+    runs-on: ${{ matrix.os }}
     strategy:
       max-parallel: 8
       matrix:
+        os: [ubuntu-18.04, macos-10.15]
         python-version: [2.7, 3.5, 3.6, 3.7, 3.8]
     steps:
     - uses: actions/checkout@v1