Extract flake8 to own Tox task

Author: hugovk

.travis.yml

@@ -3,6 +3,9 @@ dist: xenial
 
 matrix:
   include:
+    - os: linux
+      python: "3.7"
+      env: TOXENV=lint
     - os: osx
       language: generic
       env:

tox.ini

@@ -1,18 +1,25 @@
 [tox]
-envlist = py{37,36,35,py3},docs
+envlist =
+    lint
+    py{37,36,35,py3}
+    docs
 skip_missing_interpreters = True
 
 [testenv]
 passenv = DISPLAY
 alwayscopy = True
 deps =
     pytest>=4.0.2
-    flake8>=3.6.0
     numpy==1.15.4
     py37: pillow>=5.4.0
 commands =
     python -m pytest {posargs}
-    py3{7,6,5,}: python -m flake8 docs mss tests
+
+[testenv:lint]
+deps =
+    flake8>=3.6.0
+commands =
+    python -m flake8 docs mss tests
 
 [testenv:docs]
 description = build the documentation