fix : test OS changed to ubuntu-20.04

Author: sepandhaghighi

.github/workflows/test.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-latest]
+        os: [ubuntu-20.04]
         python-version: [3.7, 3.8, 3.9, 3.10.0, 3.11.0, 3.12.0]
     steps:
     - uses: actions/checkout@v2