ci: Fix CI for Python 3.7

Author: achimnol

.travis.yml

@@ -50,9 +50,6 @@ jobs:
     - python: "3.8"
       os: linux
   fast_finish: true
-  # exclude the duplicate default test stage
-  exclude:
-    - python: "3.7"
 
 notifications:
   webhooks:

src/ai/backend/client/session.py

@@ -8,10 +8,10 @@
     Any,
     Awaitable,
     Coroutine,
-    Literal,
     Tuple,
     Union,
 )
+from typing_extensions import Literal  # for Python 3.7
 import queue
 import warnings