ci: test all supported Python versions

I assume once we migrate to Python3, we want to support Python 3.6-9.
Python 3.5 will stop receiving security fixes next month, so I don't
think we need to support it.

As running the test as many times as we want is free, run them for all
supported versions to ensure capa work in all of them.

.github/workflows/tests.yml

@@ -48,7 +48,10 @@ jobs:
       matrix:
         include:
           - python: 2.7
+          - python: 3.6
+          - python: 3.7
           - python: 3.8
+          - python: '3.9.0-alpha - 3.9.x' # Python latest
     steps:
     - name: Checkout capa with submodules
       uses: actions/checkout@v2