ENH: Update supported Python versions for package builds

The minimum version has been bumped from 3.5 to 3.6. The maximum has
been bumped from 3.8 to 3.9.

Author: mseng10

.github/workflows/build-test-package.yml

@@ -131,7 +131,7 @@ jobs:
     strategy:
       max-parallel: 2
       matrix:
-        python-version: [35, 36, 37, 38]
+        python-version: [36, 37, 38, 39]
         include:
           - itk-python-git-tag: "v5.1.1.post1"
 
@@ -196,7 +196,7 @@ jobs:
     strategy:
       max-parallel: 2
       matrix:
-        python-version-minor: [5, 6, 7, 8]
+        python-version-minor: [6, 7, 8, 9]
         include:
           - itk-python-git-tag: "v5.1.1.post1"