Use ubuntu-latest in all ci tasks (#98)

Author: mib1185

.github/workflows/test.yml

@@ -36,7 +36,7 @@ jobs:
 
   pytest:
     name: pytest ${{ matrix.python-version }}
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     needs: pre-commit
     strategy:
       fail-fast: false
@@ -67,7 +67,7 @@ jobs:
 
   codecov:
     name: codecov
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     needs: pytest
 
     steps: