Move flake8 to a separate tox env

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py36,py37,py38
+envlist = style,py36,py37,py38
 skipsdist = True
 skip_missing_interpreters = True
 
@@ -10,12 +10,15 @@ setenv =
     PYTHON="{envpython}"
 commands =
     make modules
-    flake8
     py.test core hscommon
 deps =
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/requirements-extra.txt
 
+[testenv:style]
+commands =
+    flake8
+
 [testenv:WINDOWS]
 deps =
     {[testenv]deps}