Stop testing on Python 3.5

pypi has changed something in their end that is incompatible with older
Python versions. Fortunately, anything older than 3.6 is no longer that
interesting as that is the new baseline set by the oldest enterprise
distributions.

.github/workflows/test.yml

@@ -8,7 +8,6 @@ jobs:
     strategy:
       matrix:
         python-version:
-          - 3.5
           - 3.6
           - 3.7
           - 3.8