Fix "Version 3.7 with arch x64 not found" error

Author: ArtyomVancyan

.github/workflows/tests.yml

@@ -14,23 +14,24 @@ jobs:
         include:
           - python: "3.6"
             env: py36-django11
-            os: ubuntu-20.04  # 3.6 is not available on ubuntu-20.04
+            os: ubuntu-20.04  # 3.6 is not available on ubuntu-latest
           - python: "3.7"
             env: py37-django11
+            os: ubuntu-20.04  # 3.7 is not available on ubuntu-latest
           - python: "3.8"
             env: py38-django11
 
           - python: "3.6"
             env: py36-django21
-            os: ubuntu-20.04  # 3.6 is not available on ubuntu-20.04
+            os: ubuntu-20.04  # 3.6 is not available on ubuntu-latest
           - python: "3.8"
             env: py38-django21
           - python: "3.9"
             env: py39-django21
 
           - python: "3.6"
             env: py36-django32
-            os: ubuntu-20.04  # 3.6 is not available on ubuntu-20.04
+            os: ubuntu-20.04  # 3.6 is not available on ubuntu-latest
           - python: "3.8"
             env: py38-django32
           - python: "3.10"