Move linting to a separate tox env

We don't need to lint the code with every version of Python, instead add
an extra tox env which lints once with the latest supported Python version

Signed-off-by: Joshua Lock <jlock@vmware.com>

Author: joshuagl

tox.ini

@@ -4,7 +4,7 @@
 # and then run "tox" from this directory.
 
 [tox]
-envlist = py27, py35, py36, py37, py38
+envlist = lint,py{27,35,36,37,38}
 skipsdist = true
 
 [testenv]
@@ -14,8 +14,6 @@ skipsdist = true
 changedir = tests
 
 commands =
-    pylint {toxinidir}/tuf --ignore={toxinidir}/tuf/api
-    pylint {toxinidir}/tuf/api --rcfile={toxinidir}/tuf/api/pylintrc
     bandit -r {toxinidir}/tuf
     coverage run aggregate_tests.py
     coverage report -m --fail-under 97
@@ -40,3 +38,8 @@ deps =
 commands =
     coverage run aggregate_tests.py
     coverage report -m
+
+[testenv:lint]
+commands =
+    pylint {toxinidir}/tuf --ignore={toxinidir}/tuf/api
+    pylint {toxinidir}/tuf/api --rcfile={toxinidir}/tuf/api/pylintrc