TEST: Drop python 3.5 tests

Python 3.5 is no longer supported by Github actions and
 causes the tests to fail.

Author: Kai-Striega

.github/workflows/pythonpackage.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       max-parallel: 4
       matrix:
-        python-version: [3.5, 3.6, 3.7]
+        python-version: [3.6, 3.7]
         numpy-version: [1.15, 1.16, 1.17]
 
     steps: