Support pyproject.toml and use poetry with tox

Author: Cito

.travis.yml

@@ -19,11 +19,11 @@ cache:
     - $TRAVIS_BUILD_DIR/.tox
 
 install:
-  - pip install pytest pytest-asyncio pytest-cov flake8
-  - pip install .
+  - pip install "poetry>=0.12.16"
+  - poetry install
 
 script:
    - tox -e $TOXENV -- --cov-report term-missing --cov=graphql
 
 after_success:
-   - codecov
+   - codecov