Drop 3.6 and 3.7 to support Matrix

I prefer to give this package a bit longer lifespan than the main
project, since it's just data to be updated out-of-band, but for the 3.x
series I think it will likely be pretty stable, and running these on CI
just adds additional work.

.github/workflows/tests.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["2.7", "3.6", "3.7", "3.8", "3.9", "3.10", "3.11", "3.12", "3.13"]
+        python-version: ["2.7", "3.8", "3.9", "3.10", "3.11", "3.12", "3.13"]
         os: ["ubuntu-latest", "windows-latest", "macos-latest"]
         exclude:
           - python-version: "2.7"
@@ -18,13 +18,9 @@ jobs:
             os: "macos-latest"
           - python-version: "2.7"
             os: "ubuntu-latest"
-          - python-version: "3.6"
-            os: "ubuntu-latest"
         include:
           - python-version: "2.7"
             os: "ubuntu-20.04"
-          - python-version: "3.6"
-            os: "ubuntu-20.04"
     env:
       TOXENV: py