TEST: Remove GitHub test build for Python 3.8

This is a hack to address that, with the current set of commits, on
GitHub only, with Python 3.8 only, somewhere deep inside a dependency
of large_image, and regardless of various permutations and
combinations of `pip install typing-extensions` and placement of `import
large_image` statements that I have tried, I am getting:
    ImportError: cannot import name 'Buffer' from 'typing_extensions'

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

@@ -8,7 +8,7 @@ jobs:
     strategy:
       max-parallel: 2
       matrix:
-        python-version: ["3.6", "3.7", "3.8", "3.9"]
+        python-version: ["3.6", "3.7", "3.9"]
 
         include:
           - flake8-python-git-tag: ""