Need to use older Ubuntu for Python 3.6

Author: PeterJCLaw

.github/workflows/check.yml

@@ -6,7 +6,7 @@ on:
 jobs:
   test:
     name: Test
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     strategy:
       matrix:
@@ -44,7 +44,7 @@ jobs:
 
   lint:
     name: Lint
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     strategy:
       matrix:
@@ -82,7 +82,7 @@ jobs:
 
   type-check:
     name: Type Check
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     strategy:
       matrix: