Test in Python 3.6

The Python Software Foundation is supporting Python 3.6 until
2021-12-23.

CASE should test its tooling in Python 3.6 until that Python release
reaches end-of-life.

This patch is somewhat an XFAIL - I ran the unit tests in an Ubuntu
18.04 machine and found a feature used in a test is supported in
Python >= 3.7.  A follow-on patch will correct that issue.

Signed-off-by: Alex Nelson <alexander.nelson@nist.gov>

Author: ajnelson-nist

.github/workflows/ci.yml

@@ -23,7 +23,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.8]
+        python-version: [ 3.6, 3.8 ]
 
     steps:
     - uses: actions/checkout@v2