ci: stop testing on Python 3.5

since Python 3.5 is no longer supported on 2020/10/2
https://pythoninsider.blogspot.com/2020/10/python-35-is-no-longer-supported.html

Author: utgwkk

.github/workflows/test.yml

@@ -14,7 +14,6 @@ jobs:
         os: [ubuntu-latest, windows-latest]
         python-version:
           - '2.7'
-          - '3.5'
           - '3.6'
           - '3.7'
           - '3.8'