Fix flake8 plugins (#37)

* Fix flake8 plugins

* install poetry in travis

* use pip to install poetry

* move pip

.travis.yml

@@ -9,19 +9,19 @@ sudo: false
 language: python
 matrix:
   include:
-    - python: '2.7'
-    - python: '3.5'
-    - python: '3.6'
-    - python: '3.7'
-    - python: '3.8'
-    - python: '3.9-dev'
-    - python: '3.8'
+    - python: "2.7"
+    - python: "3.5"
+    - python: "3.6"
+    - python: "3.7"
+    - python: "3.8"
+    - python: "3.9-dev"
+    - python: "3.8"
       env: TOXENV=flake8
 
 install:
   - pip install -U pip wheel
   - pip install -U Cython
-  - pip install -U tox-travis tox
+  - pip install -U tox-travis tox poetry==1.1.0
 
 script:
   - tox

tox.ini

@@ -13,7 +13,9 @@ commands =
 
 [testenv:flake8]
 skip_install = true
-deps =
-    flake8
+whitelist_externals =
+    poetry
 commands =
-    flake8
+    # use poetry to install all dev dependencies
+    poetry install
+    poetry run flake8