use ubuntu-20.04 for tests involving python 3.7

Author: AhmetNSimsek

.github/workflows/siibra-testing.yml

@@ -3,7 +3,7 @@ name: '[test] unit test'
 on: [push]
 jobs:
   unit-tests:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04  # use ubuntu-latest when python 3.7 is dropped
     env:
       # unit tests should not need tokens
 
@@ -53,7 +53,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ 'ubuntu-latest', 'windows-latest' ]
+        os: [ 'ubuntu-20.04', 'windows-latest' ]   # use ubuntu-latest when python 3.7 is dropped
         python-version: [ '3.10', '3.9', '3.8', '3.7' ]
 
     steps:
@@ -92,7 +92,7 @@ jobs:
         pytest -rx e2e
 
   check-importable:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04  # use ubuntu-latest when python 3.7 is dropped
     env:
       KEYCLOAK_CLIENT_ID: ${{ secrets.KEYCLOAK_CLIENT_ID }}
       KEYCLOAK_CLIENT_SECRET: ${{ secrets.KEYCLOAK_CLIENT_SECRET }}