Bump linters to Python 3.6 (celery#4723)

* Change runtime version of linters to 3.6.

* Exclude flakeplus from this upgrade.

.travis.yml

@@ -22,33 +22,33 @@ env:
   - MATRIX_TOXENV=integration-dynamodb
 matrix:
   include:
-  - python: '3.5'
+  - python: '3.6'
     env: TOXENV=pypy-unit PYPY_VERSION="pypy2.7-5.8.0"
-  - python: '3.5'
+  - python: '3.6'
     env: TOXENV=pypy-integration-rabbitmq PYPY_VERSION="pypy2.7-5.8.0"
-  - python: '3.5'
+  - python: '3.6'
     env: TOXENV=pypy-integration-redis PYPY_VERSION="pypy2.7-5.8.0"
-  - python: '3.5'
+  - python: '3.6'
     env: TOXENV=pypy-integration-dynamodb PYPY_VERSION="pypy2.7-5.8.0"
-  - python: '3.5'
+  - python: '3.6'
     env: TOXENV=flake8
     stage: lint
-  - python: '3.5'
+  - python: '3.6'
     env: TOXENV=flakeplus
     stage: lint
-  - python: '3.5'
+  - python: '3.6'
     env: TOXENV=apicheck
     stage: lint
-  - python: '3.5'
+  - python: '3.6'
     env: TOXENV=configcheck
     stage: lint
-  - python: '3.5'
+  - python: '3.6'
     env: TOXENV=bandit
     stage: lint
-  - python: '3.5'
+  - python: '3.6'
     env: TOXENV=pydocstyle
     stage: lint
-  - python: '3.5'
+  - python: '3.6'
     env: TOXENV=isort-check
     stage: lint
 before_install:

tox.ini

@@ -55,7 +55,8 @@ basepython =
     3.5: python3.5
     3.6: python3.6
     pypy: pypy
-    flake8,flakeplus,apicheck,linkcheck,configcheck,pydocstyle,isort-check,bandit: python2.7
+    flake8,apicheck,linkcheck,configcheck,pydocstyle,isort-check,bandit: python3.6
+    flakeplus: python2.7
 usedevelop = True
 
 [testenv:apicheck]