Add Python 3.8.

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py35,py36,py37
+envlist = py35,py36,py37,py38
 skip_missing_interpreters = True
 skipsdist = True
 minversion = 3.14.0
@@ -14,13 +14,13 @@ commands_pre =
     find {toxinidir} -type f -not -path '{toxinidir}/.tox/*' -path '*/__pycache__/*' -name '*.py[c|o]' -delete
 commands =
     pytest {posargs}
-    py36,py37: black --check .
+    py36,py37,py38: black --check .
     isort --recursive --check-only --settings-path setup.cfg .
     flake8 borgmatic tests
 
 [testenv:black]
 commands =
-    black {posargs} .
+    py36,py37,py38: black {posargs} .
 
 [testenv:test]
 commands =