Use Python 3.8 for coverage testenv

Author: domdfcoding

tox.ini

@@ -218,7 +218,7 @@ addopts = --color yes --durations 25
 timeout = 300
 
 [testenv:coverage]
-basepython = python3.6
+basepython = python3.8
 ignore_errors = True
 whitelist_externals = /bin/bash
 changedir = {toxinidir}