setup.py, workflows: Require at least Python 3.9.

* Typing issues have been long-standing due to divergence between Python version
  used in different environments and are unlikely to be addressed enough to
  bring Capirca back to 3.6 compatibility. Even 3.8 is failing right now.
* Currenly, Python 3.6 and 3.7 are EOL, and 3.8-3.11 are receiving security
  fixes. At least dropping 3.6 and 3.7 makes sense at this point.

.github/workflows/main.yml

@@ -13,10 +13,10 @@ jobs:
     strategy:
       fail-fast: true
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: [3.9]
         include:
           - os-version: ubuntu-latest
-          - python-version: 3.6
+          - python-version: 3.9
             os-version: ubuntu-20.04
     runs-on: ${{ matrix.os-version }}
 

setup.py

@@ -45,7 +45,7 @@
         'License :: OSI Approved :: Apache Software License',
         'Operating System :: OS Independent',
         'Programming Language :: Python :: 3',
-        'Programming Language :: Python :: 3.6',
+        'Programming Language :: Python :: 3.9',
         'Topic :: Security',
         'Topic :: System :: Networking :: Firewalls',
     ],
@@ -56,5 +56,5 @@
         'six',
         'PyYAML',
     ],
-    python_requires='>=3.6',
+    python_requires='>=3.9',
 )