ci: remove py 3.6 and 3.7 from pipeline

Author: xeroc

.github/workflows/tox.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       max-parallel: 4
       matrix:
-        python-version: [3.6, 3.7, 3.8]
+        python-version: [3.8, 3.9]
 
     steps:
     - name: Install secp256k1

tox.ini

@@ -1,11 +1,10 @@
 [tox]
-envlist = py{36,37,38}-{linux,windows},lint,docs
+envlist = py{38,39}-{linux,windows},lint,docs
 skip_missing_interpreters = true
 
 [gh-actions]
 python =
-    3.6: py36
-    3.7: py37
+    3.9: py39
     3.8: py38, lint, docs
 
 [testenv]