added os matrix to action for more yara versions

.github/workflows/run_tests.yml

@@ -4,11 +4,13 @@ on: [pull_request]
 
 jobs:
   tests:
-    runs-on: ubuntu-latest
-
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: [3.6, 3.9]
+        os: [ubuntu-18.04, ubuntu-20.04]
+      fail-fast: false
+
+    runs-on: ${{ matrix.os }}
 
     steps:
       - uses: actions/checkout@v2