Update tox.ini

fix docs interpreter version

Author: g-k

tox.ini

@@ -66,7 +66,7 @@ commands =
     ./run_tests.sh format-check
 
 [testenv:docs]
-basepython = python3.6
+basepython = python3.8
 changedir = docs
 deps =
     -rrequirements-dev.txt