[workflows/ci.yml] Restore test support for Py 3.3, 3.4, and add 2.6

.github/workflows/ci.yml

@@ -8,9 +8,7 @@ jobs:
       fail-fast: true
       matrix:
         os: [ubuntu-20.04]
-        # TODO: python 2.6
-        # TODO: restore support for 3.3, 3.4
-        python-version: [2.7, 3.5, 3.6, 3.7, 3.8, 3.9, pypy-2.7, pypy-3.6, pypy-3.7]
+        python-version: [2.6, 2.7, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, pypy-2.7, pypy-3.6, pypy-3.7]
         python-impl: [cpython]
         ytdl-test-set: [core, download]
         run-tests-ext: [sh]